Fix Bluetooth switch status in Connected devices screen

The symptom observed is that the Bluetooth master switch on the
Connected devices page doesn't properly respond to Bluetooth turning off
via quicksettings - either turning on airplane mode or just toggling
Bluetooth.

The root cause was that MasterSwitchPreference's isChecked method would
not return the true value of whether the switch was checked - if the
control is disabled, it always just returns false. This interacts badly
with code in BluetoothEnabler - we disable the switch when the Bluetooth
state is in transition (eg becomes STATE_TURNING_OFF), and we also
attempt to avoid calling setChecked if the switch is already in the
desired state. So the switch would be checked but disabled, and we'd
avoid ever calling setChecked(false) on it.

A thorough fix would be to remove the code from MasterSwitchPreference's
isChecked method that looks at the enabled state, since enabled and
checked really should be treated as separate concerns. But given the
timeframe of MR1, we're opting for a more conservative fix of directly
accessing the switch and checking it's state, to avoid introducing bugs
in other consumers that might be depending on the current
behavior. We'll then do the thorough fix on the master branch which will
give a lot more time for any unexpected issues to be found (I audited
other usages and none seemed likely to be a problem, but it's better to
be safe than sorry).

Change-Id: I19a6c6b71e74595be3ef32a9718a430b67a89d53
Bug: 64940731
Test: make RunSettingsRoboTests