[aapt2] Add "link" option to override styles instead of overlaying.

For normal app development, the desired linking semantics are:
* styleables - take union of all definitions
* all other resources - take last non-weak definition

This differs from the semantics needed in other scenarios, where
merging/overlaying styles is desired.

Bug: 134525082
Change-Id: Iac0c43ca2ecf1f3fddc9c3367f8914c12c9258e1
Tested: aapt2_tests