Fix emulator process hanging 20+ min after CI tests finish

On API 26 google_apis, the emulator process does not exit after
receiving adb emu kill. The android-emulator-runner post-step
polls until the process dies, which never happens before the
30-minute job timeout triggers, wasting ~23 minutes of CI time.

Force-kill the qemu-system process at the end of the test script,
before the action's post-step cleanup runs. When the post-step
then checks whether the process has exited, it is already gone
and the cleanup step exits immediately.