aapt: introduce SHOW_EXTENDED_WARNINGS

a quick switch for enabling various warnings we silenced

export SHOW_EXTENDED_WARNINGS to show all silenced warnings

Change-Id: I298f36469d78e335a461f5ccfc2a66aa2976ceac
i dont feel like being "chatty"

Change-Id: I36b58871e9954161a55202115a295217f5d805ec