Do isAvailable() checks when accessing AirplaneEnabler.

This instance is only initialized if isAvailable() returns true in the
first place, so we should check for it everytime we try to access it, or
else we will result in a NPE.

Bug: 271223463
Test: atest AirplaneModePreferenceControllerTest
Change-Id: I43d35b91c86517201c8ec4f458f8c0328a9fb768