Rename CritialIssues to BackportedFixes

This part of the *API for backported fixes* which allows
app developers to check if a specific known issue is fixed a device.

Flag: excempt adding a system property.
Bug: 308461809
Test: m PlatformProperties
Change-Id: I4315e534e193d3368365c58546349261f498d774
Merged-In: I4315e534e193d3368365c58546349261f498d774