Timeout on android leaves test processes running

When xharness aborts a test run on android due to hitting the `--timeout`, it kills the `adb shell am instrument` process, but this doesn't cause the instrumentation process on the device to stop.

I don't know how the helix infrastructure handles this (whether it restarts the emulator per run, etc), but it seems like this could cause unreliability in our test infrastructure.

We should consider handling timeouts by also issuing an `adb shell am force-stop `.

Start by locating xharness's Android timeout handling and the code that launches `adb shell am instrument`. Trace what happens when `--timeout` aborts the run and determine how the target package is available there. Done means a timed-out run also stops the device-side instrumentation process; check the existing Android test infrastructure if available.

Written by the indexing model from the issue text.