Pressing enter should correspond to default button action

In shortcut customization dialogs, pressing enter should correspond to
the default button action. This is achieved by setting default focus to
the default buttons in reset and delete confirmation dialogs, and by
capturing and processing Enter key event for Add shortcut dialog. For
this key interception to work seamlessly, I've updated the
implementation of viewModel.onKeyPressed to only process key events on
ACTION_UP events as opposed to ACTION_DOWN previously. This is necessary
to ensure that when the user selects an keycombination such as
Meta+Enter in the add shortcut dialog, the dialog doesn't close
immediately in response to Enter key being pressed.

Test: ShortcutCustomizationViewModelTest
Flag: com.android.systemui.keyboard_shortcut_helper_shortcut_customizer
Fix: 381065266
Change-Id: I9f8fbc107ea3c1725ae3e756a62c95a4336c549a