Handle ActivityNotFoundException in the WM policy.

When invoking a registered shortcut for which the activity was not
found, the startActivity throws ActivityNotFoundException.  If this
exception is not caught and handled, then the shortcut key is
dispatched to the application instead of being suppressed as it
should have been.

Found while fixing Bug: 3396233

Change-Id: I0e8e9e3ba09dd403a901d5eae12839734be467ed