aapt: we dont care about uncommented public symbols

export SHOW_UNCOMMENTED_SYMBOL_WARNING to show them

Change-Id: I6e6461b54e5cd3a9c09efeb8df3172e48bbfa8e5
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