Create Configuration overrides for MCC/MNC values

SystemUI currently only shows the RAT type indicator for the active
network, which may change. At the same time, the Resources system does
not track the active mobile network, which means that
`context.getResources().getConfiguration()` may return a `Configuration`
object with an incorrect MCC/MNC for the current active subscription.

This CL creates a `MobileContextProvider` class, which will lazily
generate `Context`s with a correctly overridden MCC/MNC, so that
resource overlays properly load individual carrier iconography when the
active data subscription changes.

We also will load each individual mobile icon with the proper context,
in case there is ever a need to show the RAT type indicator for multiple
subscriptions at once.

Lastly, this CL adds a new "mccmnc" arg string to demo mode so that we
can easily validate RAT type indicator overlays.

Test: atest MobileContextProviderTest
Test: use the new `mccmnc` string in sysui demo mode to manually verify
Fixes: 240555502
Change-Id: If1f7d31a46f19f335b9545ca33638786a423ba0d