Log process exit reason and subreason as strings.

These are currently logged as integers, but this makes them harder to
use during debugging. Log them as strings instead to make them more
helpful.

Test: flipped ActivityManagerDebugConfig and observed the following log
  when swiping up to kill an activity:

  07-01 10:54:48.698   784   813 I ActivityManager: note: ProcessRecord{d3e8a8 3858:com.android.dialer/u0a63} is being killed, reason: USER REQUESTED, sub-reason: REMOVE TASK, message: remove task

Change-Id: I84c0d8f54a01f9ee6d0e774a56d35b31b875ac14