Add support to discovering activity-alias activity elements (#247)

Context https://developer.android.com/guide/topics/manifest/activity-alias-element.
Fixes https://dev.azure.com/devdiv/DevDiv/_workitems/edit/2305723.
So activity-alias allows users to change some settings before calling an activity.
The idea is that your activity is no longer the main launcher, you still have that entry, but the activity-alias' become the main launchers.
Here is a sample AndroidManifest.xml
```xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android">
<application
android:allowBackup="true"
android:icon="@mipmap/appicon"
android:roundIcon="@mipmap/appicon_round"
android:supportsRtl="true">
<activity-alias
android:name=".MainActivityAlias"
android:enabled="true"
android:icon="@mipmap/appicon"
android:targetActivity=".MainActivity"
android:exported="true">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity-alias>
<activity-alias
android:name=".MainActivityAlias2"
android:enabled="false"
android:icon="@mipmap/appicon2"
android:targetActivity=".MainActivity"
android:exported="true">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity-alias>
</application>
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.INTERNET" />
</manifest>
```
The MainActivity.cs then becomes
```csharp
[Activity(Theme = "@style/Maui.SplashTheme", LaunchMode = LaunchMode.SingleTop, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation | ConfigChanges.UiMode | ConfigChanges.ScreenLayout | ConfigChanges.SmallestScreenSize | ConfigChanges.Density)]
[Register("com.companyname.mauiappwithmultipleicons.MainActivity")]
public class MainActivity : MauiAppCompatActivity
```
Note that we do not use the MainLauncher property on the ActivityAttribute.

Author: dellis1972

src/Xamarin.Android.Tools.AndroidSdk/AndroidAppManifest.cs

@@ -329,7 +329,9 @@ void RemoveAndroidPermissions (IEnumerable<string> permissions)
 
 IEnumerable<XElement>GetLaunchableActivities()
 {
-foreach(varactivityinapplication.Elements("activity")){
+varactivities=application.Elements("activity");
+varaliases=application.Elements("activity-alias");
+foreach(varactivityinactivities.Union(aliases)){
 foreach(varfilterinactivity.Elements("intent-filter")){
 foreach(varcategoryinfilter.Elements("category"))
 if(category!=null&&(string?)category.Attribute(aName)=="android.intent.category.LAUNCHER")

tests/Xamarin.Android.Tools.AndroidSdk-Tests/AndroidAppManifestTests.cs

@@ -51,8 +51,9 @@ public void GetLaunchableActivityNames ()
 varversions=newAndroidVersions(Array.Empty<AndroidVersion>());
 varmanifest=AndroidAppManifest.Load(GetTestAppManifest(),versions);
 varlaunchers=manifest.GetLaunchableActivityNames().ToList();
-Assert.AreEqual(1,launchers.Count);
+Assert.AreEqual(2,launchers.Count);
 Assert.AreEqual(".HasMultipleIntentFilters",launchers[0]);
+Assert.AreEqual(".ActivityAlias",launchers[1]);
 }
 
 [Test]

tests/Xamarin.Android.Tools.AndroidSdk-Tests/Resources/manifest-simplewidget.xml

@@ -33,6 +33,12 @@
  <categoryandroid:name="android.intent.category.LAUNCHER" />
  </intent-filter>
  </activity>
+ <activity-aliasandroid:name=".ActivityAlias"android:label="alias">
+ <intent-filter>
+ <actionandroid:name="android.intent.action.MAIN" />
+ <categoryandroid:name="android.intent.category.LAUNCHER" />
+ </intent-filter>
+ </activity-alias>
  </application>
  <uses-permissionandroid:name="android.permission.INTERNET" />
  <uses-permissionandroid:name="android.permission.READ_CONTACTS" />

tests/Xamarin.Android.Tools.AndroidSdk-Tests/Xamarin.Android.Tools.AndroidSdk-Tests.csproj

@@ -4,7 +4,7 @@
 <!-- $(TargetFrameworks) is used to allow the $(TargetFramework) check in Directory.Build.props to work.
  If $(TargetFramework) is declared here instead, it will not be evaluated before Directory.Build.props
  is loaded and the wrong $(TestOutputFullPath) will be used. -->
- <TargetFrameworks>net6.0</TargetFrameworks>
+ <TargetFrameworks>net8.0</TargetFrameworks>
  <SignAssembly>true</SignAssembly>
  <AssemblyOriginatorKeyFile>..\..\product.snk</AssemblyOriginatorKeyFile>
  <IsPackable>false</IsPackable>