Allow bluetooth in airplane mode when in "toggleable" list.

Settings.System.AIRPLANE_MODE_TOGGLEABLE_RADIOS is a list of
radios that can be toggled while in airplane mode.  This CL
changes BluetoothEnabler to respect this.

It parallels the logic in WifiEnabler, which enables OEMs
to allow Wifi changes when in airplane mode.

Most existing devices don't include "bluetooth" in the
toggleable list, which means bluetooth will continue to be
disabled when in airplane mode for those devices.

Fixes http://b/2297314