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.
Bug 915201 - Don't depend on clang

Change-Id: I432e70695b48692d363b1a4b44b04b47004ad0f4
Bug 919982: fork device/generic/goldfish for multisim

Change-Id: I02204d268a4549799603423e8fb5ffe24ab4f454