Move Mutable{Int,Long} from libcore to framework.

No libcore dependencies on these classes remain, so they
can now move to framework which already has all of the
other classes from android.util.

After this CL topic, libcore and framework no longer have
any classes from the same package.

Bug: 67901714
Test: make droid
Test: Treehugger

Change-Id: I65871516b762d8a53ebe01697e4d92f94903bfd3