Fix issue where expectedListenerUpdate is not stable on test runs.

expectedListenerUpdates is an array of enums, which apparently does not
toString properly.
Changed the toString to generate a comma separated list of the expected
enum values.

Test: Run the BluetoothRouteTransitionTests and verify stable expectedListenerUpdate values.
Fixes: 232553291
Change-Id: I924dba0ba9f02543230e69892e92dc50aa1bcc9e