android-rebuild.sh: Run emugen_unittests.

This adds emugen_unittests as part of the normal sanity checks
performed by android-rebuild.sh.

Be sure to have the following patch applied to $AOSP/sdk/ first,
otherwise this will fail badly:

  https://android-review.googlesource.com/#/c/116642/

Change-Id: I4cc00fe79158b0cb32a75720694ece51660203c0
Merge "android-rebuild.sh: Run emugen_unittests."