Added meaningful accessibility labels for add/delete shortcut buttons

Test: manual - use talkback to navigate shortcuthelper in customize mode
and ensure that add/delete shortcut buttons have meaningful labels read
out loud.

Flag: com.android.systemui.keyboard_shortcut_helper_shortcut_customizer
Fix: 388192223
Change-Id: I5a4bf4703b48706bb091019e2a59e2e19e258cfc