OOM score management

Add 2 additional attirubte |isFocused| and |lastActivityTime| in
ActivityManager.RunningAppProcessInfo and pass them to Chrome via IPC
so Chrome can better prioritize ARC processes.

Bug: 29576205
Test: Ensure new process information is passed to Chrome.

Change-Id: Ic88d5fca336f03bd6426e65f964236130bb83fb6