Refactoring and tweaking TimeSparseArray

Tweaking the returned result from the boundary condition in
closestIndexOnOrAfter to be conceptually similar to
closestIndexOnOrBefore. This should also make it simpler to use by
callers.

Also exposing the class more broadly by moving it to android.util

Test: atest FrameworksCoreTests:TimeSparseArrayTest

Bug: 195684213
Change-Id: I44a06db8792d590be0eea4de39bc81387f2baac8