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
Test: manual
Change-Id: I7cd36585e7c292e43b5528969900c6ead2b80c0b