add shortcut dialog should receive default focus in touch mode

Previously the add shortcut dialog edit key component wasn't receiving
default focus when the dialog was triggered using touch(clicking on the
+ button on the right side of shortcut)

This has now been fixed by explicitly indicating that the component
canFocus = true rather than allow this be defined for us by the system.

Test: Manual - open  shortcut helper, enter customize mode, press the
plus button on the right side of shortcut  to open customization dialog.
Ensure the text box with "press key" prompt is focused by default.
Flag: com.android.systemui.keyboard_shortcut_helper_shortcut_customizer
Fix: 380439658

Change-Id: Ib51cb61b2b94c73b367e52074d9175319dab7f99