SF: Adding statistical mode

- Adding statistical mode for timestamp intervals.
- Moving math calculations into util class, and adding a unit test.

See go/surface-flinger-scheduler for more info and systrace links.

Test: SF tests pass. Adding new test class.
Bug: 113612090
Change-Id: I3c3a73e3d8719e47326d8a48d27683037b7beb47
10 files changed