Camera3: Add missing mTriggerMutex lock

There is thread safety problem for mTriggerMap in
Camera3Device::RequestThread::clear(). For correctness,
acquire mTriggerMutex before mTriggermap.clear().

Test: 1. Configure power saving mode to 2 seconds by
      modification software;
      2. using pyhon or monkey to turn on the screen every 4
      second;
      3. device doesn't crash any more on loop test.

Change-Id: I2f04e21dae3a9879d6cebfefb9d9c191ef3f4df4