set up ANDROID vars emerge relies on

The buildbots run `make` directly rather than loading the envsetup.sh
file.  This means none of the ANDROID vars we were using were exported.
Have the mk file recreate & pass them down.

BUG=b:26861037
TEST=`make PRODUCT-brilloemulator_arm-eng` w/python works

Change-Id: Ibdd74880b3aafa7d27eab70219ec007986f6689f
1 file changed