Improve implementation of call recording tone

Instead of using a 15-second audio file to play the call recording tone,
clip out the silence in the audio and play it once every 15 seconds.

Also start using mockito-target-extended in Telecom's unit tests so we
can mock out MediaPlayer.

Test: atest CallRecordingTonePlayerTest
Bug: 157948346
Change-Id: I7c3429733bbc7d48e29be12a72c7f15b38699ae6