BluetoothInstrumentationTests: Use an existing directory as test-filter-dir

And document why the fix is needed

Test: atest BluetoothInstrumentationTests -- --exclude-filter AdapterServiceTest#testObfuscateBluetoothAddress_PersistentBetweenAdapterServiceInitialization
Ignore-AOSP-First: cherry-pick
Fix: 255088822
Change-Id: Ia8aa2aa35881e1f23c430761b81ce39577027006
Merged-In: Ia8aa2aa35881e1f23c430761b81ce39577027006