Add per-named-item overlay ability in res xml.

Fixes 1899451.
Also errors out if you add a bag via an overlay (which would renumber
resources).  Note that you can still add elements within the bag as they are
not issued resource IDs.  So for example you can now modify the framework
themes.xml file without copying the entire file.

All bag types are now modifable except non-named bag types (how would you
address which element to replace?):
    array
    string_array
    integer_array