Squisher: Optimize Handling of APKs

* I've updated this code so that squisher no longer handles the process of making release OTAs, instead operating on $OUT/system before system images and
OTAs are created.  This has the advantage of avoiding having to unzip a just-packed OTA to optimize it before repacking it.

I first saw this done by Steven Luo and I've since adapted it to LiquidSmooth.

Signed-off-by: Chester Kener <Cl3Kener@gmail.com>