Return a default for unknown API levels (#90)

Context: dotnet/android#4873
Context: dotnet/android#4873 (comment)
When xamarin-android is built to support .NET 5, the .NET 5 install
directory contains a single `AndroidApiInfo.xml` file:
<AndroidApiInfo>
<Id>30</Id>
<Level>30</Level>
<Name>R</Name>
<Version>v11.0</Version>
<Stable>True</Stable>
</AndroidApiInfo>
`AndroidVersions`, meanwhile, is setup to read a *set* of
`AndroidApiInfo.xml` files (aad97f1) to "dynamically" compute mappings
between possible `$(TargetFrameworkVersion)` values, API-levels, and
IDs for those API levels.
When there is only one such file, if you call:
int? apiLevel = androidVersions.GetApiLevelFromId (29);
then `apiLevel` will always be `null`, because (at present)
`AndroidVersions.KnownVersions` doesn't know about API-29 or API-30.
We *could* update `AndroidVersions.KnownVersions` to contain entries
for API-29 & API-30, but doing so means that we reintroduce the
scenario that `AndroidVersions` was added to help defend against: a
need/requirement to update `AndroidVersions.KnownVersions` *every time*
a new API level was released, lest ~everything break.
We *don't* want to require `AndroidVersions.KnownVersions` updates.
To allow a .NET 5-like environment to work *without* updating
`KnownVersions`, update the various `GetApiLevelFromId()` methods to
return the incoming API level as a fallback. If `"29"` comes in, then
`29` can be returned and assumed to be a valid API level.

Author: jonathanpeppers

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

@@ -92,7 +92,8 @@ static bool MatchesFrameworkVersion (AndroidVersion version, string frameworkVer
 publicint?GetApiLevelFromId(stringid)
 {
 returninstalledVersions.FirstOrDefault(v =>MatchesId(v,id))?.ApiLevel??
-KnownVersions.FirstOrDefault(v =>MatchesId(v,id))?.ApiLevel;
+KnownVersions.FirstOrDefault(v =>MatchesId(v,id))?.ApiLevel??
+(int.TryParse(id,outintapiLevel)?apiLevel:default(int?));
 }
 
 staticboolMatchesId(AndroidVersionversion,stringid)
@@ -105,7 +106,8 @@ static bool MatchesId (AndroidVersion version, string id)
 publicstring?GetIdFromApiLevel(intapiLevel)
 {
 returninstalledVersions.FirstOrDefault(v =>v.ApiLevel==apiLevel)?.Id??
-KnownVersions.FirstOrDefault(v =>v.ApiLevel==apiLevel)?.Id;
+KnownVersions.FirstOrDefault(v =>v.ApiLevel==apiLevel)?.Id??
+apiLevel.ToString();
 }
 
 // Sometimes, e.g. when new API levels are introduced, the "API level" is a letter, not a number,
@@ -115,7 +117,8 @@ static bool MatchesId (AndroidVersion version, string id)
 if(int.TryParse(apiLevel,outvarplatform))
 returnGetIdFromApiLevel(platform);
 returninstalledVersions.FirstOrDefault(v =>MatchesId(v,apiLevel))?.Id??
-KnownVersions.FirstOrDefault(v =>MatchesId(v,apiLevel))?.Id;
+KnownVersions.FirstOrDefault(v =>MatchesId(v,apiLevel))?.Id??
+apiLevel;
 }
 
 publicstring?GetIdFromFrameworkVersion(stringframeworkVersion)

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

@@ -202,9 +202,9 @@ public void GetIdFromApiLevel ()
 Assert.AreEqual("II",versions.GetIdFromApiLevel("14"));
 Assert.AreEqual("II",versions.GetIdFromApiLevel("II"));
 
-Assert.AreEqual(null,versions.GetIdFromApiLevel(-1));
-Assert.AreEqual(null,versions.GetIdFromApiLevel("-1"));
-Assert.AreEqual(null,versions.GetIdFromApiLevel("D"));
+Assert.AreEqual("-1",versions.GetIdFromApiLevel(-1));
+Assert.AreEqual("-1",versions.GetIdFromApiLevel("-1"));
+Assert.AreEqual("D",versions.GetIdFromApiLevel("D"));
 
 // via KnownVersions
 Assert.AreEqual("11",versions.GetIdFromApiLevel(11));