Add support for aapt2 png compression level

aapt2 by default uses the slowest compression level for png
images, and it makes sense for the final release. On the other
hand, it takes ~5x longer than zlib level 6 which is still
just ~1-2% larger for Framework resources. Allowing faster
compression saves over a minute of Platform compilation time
for eng builds, so let's support it

Test: manual, build + boot Android
Flag: EXEMPT build system feature disabled without uses
Change-Id: I74462c182ab78e37647f1dfe1ec5f449458e5dc1