Add LOCAL_ASSET_DIR support (-A) to aapt2

Although performing the inclusion of assets/ at a later stage is
preferable, (since aapt just copies the files) do the easy thing and
support what aapt did. In the future perhaps we can zip up the
assets/ directories when also merging in the classes.dex.

Bug: 35461578
Change-Id: I7d7fde43333ea6455c4a1b9113bd25e1d88b7dd5
Test: manual