UIDGenerator: Misc clean up

UIDGenerator: Misc clean up

Various miscellaneous clean ups with no change to any of the
logical code. This to make the next commit have a cleaner diff
for easier review.

* Make internal millitime() non-static.

* Improve documentation and add missing @covers annotations.

* Correct getTimestampedID88() documentation to state that
  only two values need to be passed, not three.
  This is significant because the Flow extension is actually
  using this method in a subclass and passes only two values.

Bug: T94522
Change-Id: I2a0c51bea58df4cc0c253c1c10de3ac383f04c8e