Point libpng and frameworks/av to our forks

This is necessary for:

frameworks/av : https://bugzilla.mozilla.org/show_bug.cgi?id=911548 

libpng : https://bugzilla.mozilla.org/show_bug.cgi?id=915201
Change Helix on master to use Gaia & Gecko on master not v1.1.0hd branch.
use local aosp mirror

Change-Id: I9d43700cff3b1678830adf8c2906748424f17138