Bluetooth airplane listener: Check for null

Check mAirplaneHelper != null before accessing it.  Note that this can
be null if Settings.Global.RADIO_BLUETOOTH is enabled in
AIRPLANE_MODE_RADIOS.  We don't need to disable Bluetooth if it's
allowed in airplane mode.

Test: atest FrameworksServicesTests
Bug: 174254527
Change-Id: I93ae4ec6b75b7ffb8e75848d7779ab64593221bd