add bunch of new java APIs: mostly stubs or copied from AOSP

Many of these classes are only needed to be subclassed by androidx
support library, which is used in many android apps

src/api-impl/android/app/FragmentManager.java

@@ -0,0 +1,16 @@
+package android.app;
+
+public class FragmentManager {
+	
+	public Fragment findFragmentByTag(String tag) {
+		return null;
+	}
+
+	public FragmentTransaction beginTransaction() {
+		return new FragmentTransaction();
+	}
+
+	public boolean executePendingTransactions() {
+		return false;
+	}
+}