Migrate libcore code to new classes libcore.util.Mutable{Int,Long}.

The new classes are the libcore-internal versions of the
corresponding classes in android.util.

This CL migrates libcore code to use these classes instead.
The only exception are android.system.Os's static method
signatures which continue to use android.util.Mutable{Int,Long}
since those signatures are part of the Android API. The
implementation of those methods converts between those
types and the libcore-internal ones.

This further reduces the dependency of libcore code onto
android.* packages.

Test: Treehugger
Bug: 67901714

Change-Id: Id072a1e830aba92a60c405836b4f74ab6a7ce172