Mark test as flakytest

Test went from 0.1% flake to 55%, with no related CLs submitted in
that window. Mark flaky until the root cause can be found.

Fixes: 270760395
Test: atest KeyguardTransitionRepositoryTest
Change-Id: I10dce47aa2f1f92c6c0294862b02b86fffd21676
1 file changed