Move ToneGenerator recreation/release to onStart/onStop

There is no reason to do the costly ToneGenerator initialization
inside onResume, which gets called during the Dialer -> InCallUI
transition. Move it to onStart/onStop so it only happens when Dialer
is actually in the background.

Measurement of time taken for InCallActivity start -> onResumed, 10 runs

Before: Average 541.9ms SD: 152.4ms
After: Average 350.5ms SD: 81.1ms

Bug: 18373617
Change-Id: I38fa5e840708b991bf5d6628bb8873cd2399013b