Block addition of custom shortcuts

- If overlapping with system shortcuts
- If ovelapping with bookmarks
- If no modifierState provided for key based shortcut
- If using a system keycode for key based shortcut

DD: go/customizable_shortcuts
PRD: go/custom-kb-shortcuts

Bug: 365064144
Test: atest InputTests
Flag: com.android.hardware.input.enable_customizable_input_gestures
Change-Id: I69dd162f3318a84123b748d5ee5b9074844d8790