Fix issue #2174566: HOT STABILITY: NPE in activity manager

- Don't crash if the service's app process doesn't have a thread (this
  can happen if we are currently starting the process and then try to
  start the service again).
- Be more robust about deciding to start a service's process: try each
  time it is started; the startProcess call will take care of ignoring
  it if the process is already started.
- Fix some issues where we would leave dead processes on the low memory
  list.

Change-Id: I490e01ba7b45adc191bab7ace377b6873e284897