Refresh constants when MCC/MNC changes in PowerUI

The PowerUI in SystemUI decides whether to show high temperature
warnings to the user based on resources defined for mnc/mcc
configurations. PowerUI only loads the relevant constants once
at startup, meaning any SIM changes that come after the initialization
are not visible. Sometimes, these changes are the first initialization
of mnc/mcc.

PowerUI now listens to configuration changes and updates its internal
state when mnc/mcc changes occur.

Bug: 62845934
Test: manual (put SIM in device and reboot)
Change-Id: Id6a13017924f0367594aa29419aac59ac8b7c157