Add ADB command to wait for system server crash

"adb shell svc system-server wait-for-crash" will block until the process dies.

Bug: 124022170
Test: Manual
- Run "adb shell svc system-server wait-for-crash" on multiple terminals
- Make sure open FDs aren't increasing by checking /proc/`pid system_server`/fd
- adb shell killall system_server
- All the "adb shell svc system-server wait-for-crash" should finish.

Change-Id: I203d001cd296d506ebf9f5a70b97e05e5769961a