[Autofill CredMan]: Autofill changes for CredMan integration

For views labelled with for isCredential, disable autofill.
Manual request should still continue to work.

This CL also refactors feature flags to a common class.

BUG: 261045584
Test: atest android.autofillservice.cts.dialog.LoginActivityTest#testCredentialManagerIntegration --iterations=10. Also tested manually with the formgen app with custom changes.
Change-Id: Ic2b71b2c027d5fc34006b1660d8ee13ce69aac4c