Add @FlakyTest annotation to LegacyProviderMigrationTest am: 7387c081c1

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/providers/MediaProvider/+/13356025

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I354a09c94606ed18a5a8d30288abaa8747126594