Make BluetoothInstrumentationTests preparer multi-user aware

The current target_preparer only calls "svc bluetooth enable|disable"
under user 0. These calls will fail for multi-user builds where
Bluetooth is being ran/tested under a user that isn't user 0.

This change will make the preparer run the command under the current
user instead. It should solve considerable test flake and test coverage
statistic problems on multi-user builds.

Tag: #stability
Bug: 203807060
Test: atest BluetoothInstrumentationTests -- run on a car with user 10
Change-Id: I294ff9f2118e62824b6a2ee045b371434003a01a