@Ignore ActivityTransitionTest#testFinishPostponed

This test has been flaky for 2 years. Between U and V it will
be removed and reimplemented in another module (b/278881857)

Test: atest 'android.app.usage.cts.ActivityTransitionTest#testFinishPostponed'
Bug: 279524568
Fix: 293496765
Bug: 278881857
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:bbeb43064354ab376e56b66e8c064dd5b46b9a6a)
Merged-In: I1b6840655063f779e2024e9e14a4002056ffab9d
Change-Id: I1b6840655063f779e2024e9e14a4002056ffab9d