Disable sim toggle switch if preference is disabled

Normally, if a SwitchPreference is disabled, the Switch
GUI element is also disabled. MultiSimEnablerPreference added
an "explicitly disabled" flag to allow a client to explicitly
disable the switch. However, as written, the code was looking
at the "explicitly disabled" flag only, and ignoring whether
or not the preference was disabled. This change fixes that.

JALEBI-139

Signed-off-by: Edward Falk <efalk@cyngn.com>
Cherry picked from cm-12.1, eb04e7cc

Change-Id: I33de46fa19808075756933fbbb32e53e32ca8eab