Update onBackPressed() function signature

A follow-up to ag/20093581 (see b/244635782#comment5).
Due to a refactor, StatusBarKeyguardViewManager::onBackPressed
no longer accepts or returns a boolean.
This parent class (and the child in vendor/google_arc)
are being updated simultaneously.

Bug: 244635782
Test: Passed presubmit
Change-Id: Idd16e938162bf50f528efd3934a116f9fde9d60d