Run 'emugen' test suite as part of android-rebuild.sh

This patch modifies android-rebuild.sh to ensure that the 'emugen'
test suite is run after the binary is rebuilt. This will help catch
regressions more easily.

NOTE: This requires the followind sdk/ patch to be submitted:

          https://android-review.googlesource.com/#/c/112552

Change-Id: I0ee954788f9bff7021fa6a168b9d09791b758c7f
distrib/update-emugl-sources.sh: New script.

This script can be used to update the sources of the GPU emulation
encoder libraries under $AOSP/device/generic/goldfish/opengl/

Use it whenever the 'emugen' implementation changes.

Change-Id: I1e3f430dd3d241ed9cd3bb065bbcfe8bc66446a2
Merge changes I1e3f430d,I0ee95478

* changes:
  distrib/update-emugl-sources.sh: New script.
  Run 'emugen' test suite as part of android-rebuild.sh