SF: region sampling scheduling optimization

Schedule the region sampling thread more wisely by estimating when
the next invalidate is going to happen. This makes the region sampling
to be scheduled often at times where the main thread is idle, without
eating the budget in an invalidate message.

Bug: 181983990
Test: SF unit tests
Test: observe systrace
Change-Id: I1faca3aa7f882ed7c69e77e6a0877c10d57f0f1c