Add rules to handle asset resources.

AAPT2 ignores assets in overlay resource inputs, so separate rules are
required to extract and merge assets from dependencies.

Test: app_test.go
Test: Added assets to Settings-core for testing
Test: Confirmed Settings.apk contains them with this change.
Fixes: 146655310
Change-Id: Iab8bc61b719541dae64f0e3502bc9cb45a353687