Add default theme for flicker test app

Otherwise its WidowManager.LayoutParams#windowAnimations is 0, then
it won't play animation (TransitionAnimation#loadAnimationAttr
returns null) so the the surface of closing app may be updated to
invisible at random timing that causes the test flaky.

Use darker gray as window background color to make it easier to
distinguish the visual result between home, starting window (gray)
and app content (orange).

Bug: 216297992
Bug: 213852103
Test: com.android.server.wm.flicker.launch.OpenAppColdTest
Change-Id: I3bb073dfa52484a63168b3aeb6e7f5f43bd02aac