[Android] Use the UI thread for VSyncMonitorTest vsync requests
VSyncMonitor is not thread-safe, and is not intended to be. When a vsync
request is made from a non-UI thread, there is a race where both a
synthetic and frame-based callback can fire for the same request. Avoid
this by making sure VSyncMonitorTest only requests updates from the UI
thread, representative of production code behavior.
BUG=498007
Review URL: https://codereview.chromium.org/
1166223003
Cr-Commit-Position: refs/heads/master@{#333580}