clean up notification of FilterCallbackScheduler when stopping

I am adding two minor tweaks to change
I2b9d45fa7df3f2490893cc24d98c0e0baaaf0c2c which notifies the condition
variable when trying to join the thread:
    1. the mutex lock does not need to be held to notify the condition
    variable.
    2. adding isRunning check to existing if condition in
    threadLoopOnce to condense the code.

Bug: 183057734
Test: atest android.media.tv.tuner.cts
Change-Id: Ia35e645bddd5005218e8e017a22b87b349997658