Ensure that activity options menu items reflect the bound service state.

- Call supportInvalidateOptionsMenu() when the service binding completes.

- Implement onPrepareOptionsMenu() in derived classes to
  set the enabled state of menu items that require a service connection
  based on the service connection state.
Start handling a nullable service binding in NowPlayingFragment.

- Mark mService as @Nullable.

- Pass the bound service as a parameter to onServiceConnected().

- Pass the bound service as a parameter to maybeRegisterCallbacks().

- Use onPrepareOptionsMenu() to set the state of options menu items.
  Call getActivity().supportInvalidateOptionsMenu()
Verify that the return from getService() is not null.
Propogate the return value from changeVolumeBy().

This returns false if the service is is not bound.