Disable failing SystemDreamTest

This test is currently very flaky, disabling for now until we can investigate why the broadcast isn't being received when dream switch happens.

Bug: 325686454
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:38ec864e2a2645e239fb1db8a87f5f2606a73d9a)
Merged-In: Iaaee615c293e51c6e608d2fc718d66f1fb3ab7cd
Change-Id: Iaaee615c293e51c6e608d2fc718d66f1fb3ab7cd