[Mobile] Wrap AccessibilityContentDescriptions in a static getter

The content descriptions are mapped to level by their position in the
PHONE_SIGNAL_STRENGTH array, and thus are susceptible to out of bounds
index exceptions.

This change adds accessor methods to AccessibilityContentDescriptions
so that they can be checked (it's the responsibility of the caller to
check against a 0 return value which will throw if trying to get the
resource).

We also make the content description nullable and handle the resid=0
case.

Test: MobileIconViewModelTest
Bug: 328547883
Flag: NONE
Change-Id: Ifa55ad8184a5337390d9fa245311b4e9513c0233