Rebase ThemeImpl rather than reallocate memory

Memory churn is high when swapping the ResourcesImpl of a Resources
object. Each time Resources#setImpl is invoked, all themes based on
that Resources object are assigned new ThemeImpl objects that are
created using the new ResourcesImpl.

ThemeImpls can only belong to one Theme object, so the old
implementation is discarded and the theme takes ownership of the new
ThemeImp.

This creates performance problems when framework overlays are toggled.
Toggling overlays targeting the framework causes all themes across all
processes to recreate and reallocate all of their themes. By rebasing
the ThemeImpl on the new ResourcesImpl without deallocating the native
theme memory, we reduce churn and produce less garbage that needs to
be garbage collected.

Bug: 141198925
Test: atest libandroidfw_tests
Test: atest ResourcesPerfWorkloads
Change-Id: I03fb31ee09c9cfdbd3c41bcf0b605607dab54ed7