Make RollbackInfo#getRollbackImpactLevel SystemApi

We would be using this API from RollbackPackageHealthObserver which
would be moved to the CrashRecovery module

Bug: 354693320
Test: atest RollbackManagerTest
Flag: android.crashrecovery.flags.enable_crashrecovery
Change-Id: Iaa61ecf016fde1d9a0502916e5ea9da74669dd7c