Wait for MediaStore idle state to reduce the test flake am: 51fd45cd am: 6719246f am: c0871faf am: 5f601875

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



Change-Id: I61ed381aa7c8595a987bd6ecf842ca58e52f603c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>