Add support for cutom avds

This introduces the "avd" fixture which launches an emulator. Individual
modules can specify their own specific avd under which they want to run
v.s. the default.

The introduction of the "avd" fixture required a refactor to the
eliminate all the pytest.emulator globals.

Change-Id: I45e5b90db2225f0f6e4dfcedca54b53bc679dddc
23 files changed