Add @FlakyTest annotation to LegacyProviderMigrationTest

These tests are flaky because they reset vold, which now has to mount
all volumes. Tests remain flaky even if we poll for 30 seconds to see
externalStorageState=MEDIA_MOUNTED. Hence, adding @FlakyTest annotation
to indicate that the test can be flaky.

Bug: 176977253
Test: Presubmit.
Change-Id: I9a47c4928d18bfe33cd7f181bd4942e45720d7d5
1 file changed