Deprecate android.util.Mutable*.

Since http://r.android.com/565744 , these classes do not appear
in the signature of any other Android API. There are no plans to
make them part of any future API signatures.

They do not provide useful abstractions because:
 - They lack encapsulation and functionality (value is nonfinal;
   lack of equals/hashCode and toString).
 - It's trivial for apps to implement similar types for their
   internal use.

Only three of these eight classes (Mutable{Int,Long,Boolean}) are
used internally in Android; most of these usages could be written
better with named, more specific classes.

Therefore these classes do not pull their own weight on the API
surface of android.util.

This CL deprecates all eight classes in preparation for removing
them from the Android API surface at some point in the future, and
to allow the unused classes to be removed entirely.

Bug: 71546998
Test: Treehugger

Change-Id: I1cc1eb5ca9c36749bbb9a233d60036f6319bf2d3