Make it easier to serialize emulator access through a setprop/getprop locking pattern

[rib]

To help simplify being able to serialize running of tests against an emulator (regardless of whether multiple tests are started within a single process or multiple processes) it could be good it were easy to "lock" the emulator by storing the pid of a test runner in some well known system property via setprop and being able to poll until the process exits before attempting to set yourself as the next runner.