Close AirplaneModeEnabler once lifecycle classes are destroyed.

The AirplaneModeEnabler is a ContentObserver, and it must be closed when
the object containing it is deleted. Otherwise, it will continue to
exist and cause a memory leak.

Test: atest -c SettingsUnitTest
Bug: 177265744

Change-Id: Idd48b601be1133ee02010e23d8dcb394d1e29bd3