Do not include assets in autogenerated RROs

https://r.android.com/3390619 changed RRO enforcement by creating the
autogenerated RROs in Soong, instead of creating the RROs in make. These
autogenerated modules are created in the same directory as the base app,
and therefore would include asset files in `assets` subdirectory (if it
exists).

This CL adds a `doNotIncludeAssetDirImplicitly` to `aapt2Flags` to exclude the assets
from aapt2 linking.

Bug: 382345660
Bug: 375277835
Test: in internal, lunch <pixel>-trunk_staging-userdebug
Test: m out/target/product/$pixel/vendor/overlay/SystemUIGoogle__$pixel__auto_generated_rro_vendor.apk
Test: unzip -l apk and verified that assets dir is not present
Change-Id: Ie7c72bc77c277fabec0ba19c238dcc47a985c304