bind/java: reenable asset access.

This is done by moving app.Context to internal/mobileinit,
introducing mobileinit.SetCurrentContext and,
making bind/java depend on it.

TODO: check gomobile bind's proguard rule - context lookup
was implemented through reflection on android.app.AppGlobals class.

Change-Id: Ieb6ad503eeef8c2c1c5836a21c667938c5a701a2
Reviewed-on: https://go-review.googlesource.com/12279
Reviewed-by: David Crawshaw <crawshaw@golang.org>

bind/java/SeqTest.java

@@ -17,6 +17,12 @@ public class SeqTest extends AndroidTestCase {
   public SeqTest() {
   }
 
+  public void testAssets() {
+    String want = "Hello, Assets.\n";
+    String got = Testpkg.ReadAsset();
+    assertEquals("Asset read", want, got);
+  }
+
   public void testAdd() {
     long res = Testpkg.Add(3, 4);
     assertEquals("Unexpected arithmetic failure", 7, res);