Allow overriding emulator

[menny]

Overview

As the current emulator-launcher implementation is implemented for Python2 and does not allow running on Firebase, I suggest here a way override the launcher target with a different one that the user supplies.

Proposed Changes

Provide a label in the android_test_repositories init macro. The value of the new emulator_launcher_target argument will be bind to a variable which will then be used by the alias target unified_launcher_head.