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

After preceding CLs removed remaining libcore dependencies,
there are now no longer dependencies on Mutable{Int,Long}
in libcore. This CL moves those classes into framework,
where the rest of android.util already lives. After this CL,
libcore and framework contain distinct packages (android.util
was the only package that previously existed in both).

Bug: 67901714
Test: Treehugger

(cherry picked from commit 3510dc63)

Change-Id: I696a48339f8a02d7c10e0460068ba560f86ac67e
Merged-In: I696a48339f8a02d7c10e0460068ba560f86ac67e