[nativeaot] set $HOME and $TMPDIR (#9889)

Context: #9846
Context: https://github.com/dotnet/runtime/blob/d09a42b9149e14a37c8ef8363cdb7e17bb922c37/src/libraries/System.Private.CoreLib/src/System/IO/Path.Unix.cs#L83

`System.IO.Path.GetTempPath()` doesn't work under NativeAOT as seen
by the test failure:

	Mono.Android.NET_Tests, Android.RuntimeTests.InputStreamInvokerTest.InputStreamTest / Release
	System.IO.DirectoryNotFoundException : IO_PathNotFound_Path, /tmp/
	   at Interop.ThrowExceptionForIoErrno(Interop.ErrorInfo, String, Boolean) + 0x11
	   at Interop.CheckIo(Int64, String, Boolean) + 0x5d
	   at System.IO.Path.GetTempFileName() + 0x14d
	   at Android.RuntimeTests.InputStreamInvokerTest.InputStreamTest() + 0x18
	   at libMono.Android.NET-Tests!<BaseAddress>+0x1483023
	   at System.Reflection.DynamicInvokeInfo.Invoke(Object, IntPtr, Object[], BinderBundle, Boolean) + 0xf3

On Unix-like systems, the BCL just reads `$TMPDIR` and falls back to
`/tmp/` if it doesn't exist.  `$TMPDIR` is *not* set by default in
Android applications.

To fix this, we can set `$TMPDIR` to `context.getCacheDir()` just as
we do in .NET for Android:

  * https://github.com/dotnet/android/blob/04cd6f40989a8fad30d9a9759aec02d540cc9478/src/java-runtime/java/mono/android/MonoPackageManager.java#L53-L54
  * https://github.com/dotnet/android/blob/04cd6f40989a8fad30d9a9759aec02d540cc9478/src/native/mono/monodroid/monodroid-glue.cc#L1444-L1445

I think we can simply read these values in Java at startup, and call
[`Os.setenv()`][0] to set them.  There does not seem to be a need to
pass these paths into managed code.

The `Android.RuntimeTests.InputStreamInvokerTest.InputStreamTest()`
test now passes with these changes.

I fixed `$HOME` as well, as it will be used by other APIs in the BCL.

[0]: https://developer.android.com/reference/android/system/Os#setenv(java.lang.String,%20java.lang.String,%20boolean)

Author: jonathanpeppers

src/Xamarin.Android.Build.Tasks/Resources/NativeAotRuntimeProvider.java

@@ -1,5 +1,7 @@
 package net.dot.jni.nativeaot;
 
+import android.system.ErrnoException;
+import android.system.Os;
 import android.util.Log;
 import net.dot.android.ApplicationRegistration;
 
@@ -24,6 +26,18 @@ public void attachInfo(android.content.Context context, android.content.pm.Provi
         if (context instanceof android.app.Application) {
             ApplicationRegistration.Context = context;
         }
+
+        // Set environment variables
+        try {
+            String filesDir = context.getFilesDir().getAbsolutePath();
+            String cacheDir = context.getCacheDir().getAbsolutePath();
+            Os.setenv("HOME", filesDir, true);
+            Os.setenv("TMPDIR", cacheDir, true);
+        } catch (ErrnoException e) {
+            Log.e(TAG, "Failed to set environment variables", e);
+        }
+
+        // Initialize .NET runtime
         JavaInteropRuntime.init();
         // NOTE: only required for custom applications
         ApplicationRegistration.registerApplications();