Backbone of the new aapt2 strip command.

The strip command takes an APK as argument and a --target-densities flag which
contains the list of screen densities for which to optimize the APK. All
the resources that would be unused for a device having the given
densities are stripped from the APK leaving only the ones actually being
used.

Test: Manual.

Change-Id: I68368b00876a18ccdbbfc0a8dd265bd361117933