ShortcutHelper: fix UnsupportedOperationException when merging shortcuts

The crash can happen when the received app shortcut groups is an empty
list that was instantiated using Collections.emptyList(). Calling add()
or addAll() on it will crash, as it isn't implemented.

Fixes: 345025561
Test: KeyboardShortcutsTest
Flag: NONE simple bug fix
Change-Id: I321d531057856bba0600dbf9407521790d5304e9