PSS: Use a fake WindowMetricsProvider in screenshot tests

This is an attempt to reduce flakiness of the test. It seems that the
values returned from WM are not stable in between tests.

Change-Id: Id04546fb72f2ec9d00eb4719e27cfa145cac1e1e
Fixes: 302599732
Test: TaskPreviewSizeProviderTest.kt
Test: WindowMetricsProviderImplTest.kt
Test: atest SystemUIGoogleScreenshotTests:MediaProjectionAppSelectorScreenshotTest