Support changing style parent in overlays

Package overlays makes it possible for vendors to tweak the look of
the platform and the applications without touching any platform or
application code directly. This makes package overlays an important
mechanism in the Android build system.

There is currently a limitation that forbids changing the parent of a
style. If vendors could change the parent of e.g. ‘CalendarTheme’
from Android’s vanilla ‘Theme’ to ‘VendorTheme’, then vendor
specific adjustments could be done without changing any code directly.

From looking at the code it can be seen that the parent value of a
style is stored temporarily in ResourceTable::Entry::mParent while
overlays are gone through in buildResources(), and processed (in
ResourceTable::Entry::assignResourceIds()) at first after all overlays
have been handled, so there aren’t any obvious reasons why changing
parent in an overlay should be forbidden.

Change-Id: I5969bb8aab90df437e1967fc504cc0da79107d13