BouncyBall: Test at default frame rate

We used to try to force the app to use 60Hz for testing.

However, our emphasis is on testing the default user experience,
and many devices default to more than 60Hz.  For those devices, it
makes more sense to test at the default frame rate.  Although we
do require a minimum of 60Hz.

As part of this change, instead of hard-coding the number of
initial frames we ignore, we make them based on time and adjust
it per the frame rate.

Bug: 436919584
Bug: 408044970
Test: Ran app on devices that default to 60Hz and 90Hz, and things worked as expected
Flag: EXEMPT for test app
Change-Id: Ia56d06cf8edaf0e8b1b61801c35d628b10fd7bf2