Wait for MediaStore idle state to reduce the test flake am: 51fd45cd am: 6719246f am: aaffa6f0 am: b3d76c22

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



Change-Id: Ic556b8d4e6d0edfd40a4baa64cd44b1b11fca2dd
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>