Clean up some unnecessary methods of ActivityRecord

- The safelyDestroy is only called by test.
- occludesParent == fillsParent && !finishing
  And keep consistent way to access finishing state.
- Use translucent theme to make activity translucent. So it
  is closer to real case than mocking method.

Bug: 163976519
Test: testCheckOpaqueIsLetterboxedWhenStrategyIsApplied
Change-Id: I38c2472b4ab972d66f280a6933599a6e48b28eaf