Fix NotificationProgressBar tracker updating issues.

This fixes issue 2&3 mentioned in the bug description.
  - Issue 2 is fixed by the changes around `setTracker`.
  - Issue 3 is fixed by the changes around
    `onMaybeVisualProgressChanged`.

Also only `clipRect` when we intend to, i.e., when `mTrackerHeight` is
set to non-zero.

Also cleaned up `setTracker` to return early when `tracker == mTracker`.
Plus covered a couple more cases for `requestLayout`. (diff patch 2 vs
1).

Flag: android.app.api_rich_ongoing
Fix: 379359830
Test: manual - steps for issue 2&3 in the bug description.
Change-Id: Ie62a90649d5b0ce54f7d0cefcad1da4f3012b028