Add support to discovering activity-alias activity elements

[dellis1972]

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 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

[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.

[pjcollins]

This looks good to me, but can we add a test? I think we may already have a good spot to add to https://github.com/dotnet/android-tools/blob/main/tests/Xamarin.Android.Tools.AndroidSdk-Tests/AndroidAppManifestTests.cs#L49