Fix testOverrideSplashscreenTheme flaky by preventing from double launch

The test flaky because there could start activity twice before first
activity destroyed, so there didn't set the theme correctly. Add an
extra callback to preventing from double start activity.

Fixes: 194120010
Test: run "atest SplashscreenTests" several times
Change-Id: I510f1e9190c69f2a4f3d0a35ccb7fa96dbee1e32
1 file changed