[flexiglass] Fix lost focus on PIN input

When entering a PIN via keyboard, the keyDown event for the enter key resulted in the PinBouncer's VerticalGrid to lose focus and instead focus the next element, which is the first digit in the pin pad. Entering additional numbers still worked, but the enter key on that digit was consumed (resulting in the digit being entered). This prevented submitting the PIN by pressing enter again.

With this change, we also consume the keyDown event for the enter key and therefore don't pass on focus. Consecutive key presses for the enter key are still handled via PinBouncerViewModel's onKeyEvent() like expected then.

Fixes: 413013914
Flag: com.android.systemui.scene_container
Test: atest PinBouncerViewModelTest
Change-Id: Ia56187f23c436df661a43f540f9d42753aca3d24