AwesomeAction: check if ACTION_ASSIST intent can be handled

This will avoid a force close if no handler is available.

Fixes: http://bugs.aokp.co/issues/95

Change-Id: I041e8739ff9271cc02a73009e3def0585b00ffbc
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>