Replaced auto-fill by autofill to keep it consistent with API style.

This change will affects 2 types of apps: autofill service implementations
and apps that use autofill APIs.

Since just the former is known to be used at the moment, we're not trying
to keep backward compatibility with the latter.

Bug: 35956626
Test: CtsAutoFillServiceTestCases pass
Test: android.provider.SettingsBackupTest pass

Change-Id: Ia720083508716deae9e887f9faa7ae7c5a82f471