Fix possible NPE in TransparentPolicy

Even if the NPE would require TransparentPolicy#isRunning()
to be false which is not possible if also
`mTransparentPolicyState.mFirstOpaqueActivity` is null,
we use Optional.ofNullable() to prevent problems in case
the isRunning() method is mocked in tests.

Also remove unused methods.

Flag: EXEMPT small fix
Fix: 352868162
Test: atest WmTests:TransparentPolicyTest

Change-Id: Idbefec0e7290507f0b40347b2809d9dca40cf94b