Android emulator mozharness scripts (android_emulator_unittest.py, etc) assume the location of emulator avds is always in /home/cltbld/.android, unless in developer mode. That won't be convenient on taskcluster. Better to use a mozharness configuration setting. (Curiously, there is a setting in the 4.3 config, but it isn't used!)

This allows us to configure the path to the avd, even when not in developer_mode. Does no harm to current jobs: https://treeherder.mozilla.org/#/jobs?repo=try&revision=4dbae43e8ecc I'll make use of this in bug 1270544.