Minor UI Changes

+ UI Component showing selected key combination will now be focused by
  default
+ Set shortcut button will be enabled when user presses some
  keyCombination since we cannot yet validate key combination without
attempting to set shortcut

Test: Manual, open add shortcut dialog and ensure the changes above are
in effect.
Flag: com.android.systemui.keyboard_shortcut_helper_shortcut_customizer
Bug: 373638584

Change-Id: I79008ba1d1155e4a80772b4695c77849e5973a7f