Simplify SystemClock and FakeSystemClock

- Remove hidden methods from SystemClock (we shouldn't be using them
anyway).
- Change FakeSystemClock to keep all of its members consistent with one
another.
- Change FakeSystemClock to only be able to move forward in time
- Simplify listener interface for FakeSystemClock

Test: atest
Change-Id: Idd42c03a3f7b992a4c1b4a626f3ad66f457e2d89