cros: Make ScreenLockerTester have a higher-level synchronous API that handles waiting

- Simplify helper code used with ScreenLockerTest.TestFullscreenExit
- Simplify BrailleDisplayPrivateAPIUnitTest.KeyEventOnLockScreen, remove MSAN
  restriction since views-lock screen is significantly faster to load

Bug: 899777
Change-Id: I0cf3c50c562289c42513b8509a86f901caf93035
Reviewed-on: https://chromium-review.googlesource.com/c/1327543
Commit-Queue: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613321}
6 files changed