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
 		public int? GetApiLevelFromId (string id)
 		{
 			return installedVersions.FirstOrDefault (v => MatchesId (v, id))?.ApiLevel ??
-				KnownVersions.FirstOrDefault (v => MatchesId (v, id))?.ApiLevel;
+				KnownVersions.FirstOrDefault (v => MatchesId (v, id))?.ApiLevel ??
+				(int.TryParse (id, out int apiLevel) ? apiLevel : default (int?));
 		}
 
 		static bool MatchesId (AndroidVersion version, string id)
@@ -105,7 +106,8 @@ static bool MatchesId (AndroidVersion version, string id)
 		public string? GetIdFromApiLevel (int apiLevel)
 		{
 			return installedVersions.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, out var platform))
 				return GetIdFromApiLevel (platform);
 			return installedVersions.FirstOrDefault (v => MatchesId (v, apiLevel))?.Id ??
-				KnownVersions.FirstOrDefault (v => MatchesId (v, apiLevel))?.Id;
+				KnownVersions.FirstOrDefault (v => MatchesId (v, apiLevel))?.Id ??
+				apiLevel;
 		}
 
 		public string? GetIdFromFrameworkVersion (string frameworkVersion)

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));