Wait for MediaStore idle state to reduce the test flake am: 51fd45cd am: 6719246f am: c0871faf

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/DocumentsUI/+/18796607



Change-Id: Ie827e424ac4fafd9abbf9f202c3e575bfd8dbe60
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>