A11y now announces error messages when customizing a shortcut

Test: Manual - with talkback on, try to customize a shortcut using a
combination that's already in use. ensure talkback announces error
message
Flag: com.android.systemui.keyboard_shortcut_helper_shortcut_customizer
Fix: 390304655

Change-Id: I37f90fb8577ec988bfa14939893c3c26e2fcb602