[Java.Interop] optimize JniTypeManager.AssertSimpleReference() (#1001)

We noticed `dotnet trace` output was showing:
21.28ms (0.45%) java.interop!Java.Interop.JniRuntime.JniTypeManager.AssertSimpleReference(string,string)
This code path was introduced by a new feature in 1f27ab5.
For now, I think we can rewrite this to use the
[`string.IndexOf(char)`][0] overload of `string.IndexOf()`, as well
as the `string` indexer instead of `string.StartsWith()` and
`string.EndsWith()`.
After these changes, I get a better time:
1.21ms java.interop!Java.Interop.JniRuntime.JniTypeManager.AssertSimpleReference(string,string)
We may have just *moved* the location that ICU is loaded, but this
change is good regardless.
[0]: https://docs.microsoft.com/en-us/dotnet/api/system.string.indexof?view=net-6.0#system-string-indexof(system-char)

Author: jonathanpeppers

src/Java.Interop/Java.Interop/JniRuntime.JniTypeManager.cs

@@ -115,14 +115,20 @@ void AssertValid ()
 
 internalstaticvoidAssertSimpleReference(stringjniSimpleReference,stringargumentName="jniSimpleReference")
 {
-if(jniSimpleReference==null)
+if(string.IsNullOrEmpty(jniSimpleReference))
 thrownewArgumentNullException(argumentName);
-if(jniSimpleReference!=null&&jniSimpleReference.IndexOf(".",StringComparison.Ordinal)>=0)
+if(jniSimpleReference.IndexOf('.')>=0)
 thrownewArgumentException("JNI type names do not contain '.', they use '/'. Are you sure you're using a JNI type name?",argumentName);
-if(jniSimpleReference!=null&&jniSimpleReference.StartsWith("[",StringComparison.Ordinal))
-thrownewArgumentException("Arrays cannot be present in simplified type references.",argumentName);
-if(jniSimpleReference!=null&&jniSimpleReference.StartsWith("L",StringComparison.Ordinal)&&jniSimpleReference.EndsWith(";",StringComparison.Ordinal))
-thrownewArgumentException("JNI type references are not supported.",argumentName);
+switch(jniSimpleReference[0]){
+case'[':
+thrownewArgumentException("Arrays cannot be present in simplified type references.",argumentName);
+case'L':
+if(jniSimpleReference[jniSimpleReference.Length-1]==';')
+thrownewArgumentException("JNI type references are not supported.",argumentName);
+break;
+default:
+break;
+}
 }
 
 // NOTE: This method needs to be kept in sync with GetTypeSignatures()