Updated Add custom shortcut dialog string

Description text was previously "Press key to assign shortcut"
And is now: "To create this shortcut, press the Action key and one or
more other keys together"

Error message was previous "Key combination already in use. Try another
key" and is now: "Key combination already in use, Try another
combination"

Test: Manual - check that strings in the add custom shortcut dialog are
updated as above.
Flag: com.android.systemui.keyboard_shortcut_helper_shortcut_customizer
Fix: 388430701

Change-Id: I69b1756f1ec449587e69a6e55de2156ed10d862f