squisher: update & enhance for cm-11.0

Based on pershoot's modifications from Samsung P3 config

* Modified opticharger, squisher for CM-11.0 build system
* Added support for pngquant (lossy png compression) and
  set it as the preferred tool
* Disabled .asrc compression (negative CPU & memory usage impact)
* Modified script to process all system & extras folders that
  potentially contain apk files.

Change-Id: I87e99f7142ffbec805791236057ee109fe881f58