Reduce test flakyness

Sometimes the shared preference is not entirely cleaned up at start of
the test. Adding this should definitly prevent it from ever happening
again

Ignore-AOSP-First: Migration is only on tm-qpr-dev
Bug: 241152450
Test: atest BluetoothInstrumentationTests#DataMigrationTest
Change-Id: Iff81ee692ebfdfb01fc196574de0a69bd6fd2de8