cmparts: Change how parts are launched

 * Currently we're requiring a specific action with the part name
   stored in the extras. This is a bit unwieldy and we can just
   encode the part name into the action since we only ever use
   explicit intents. It also will help us integrate search in the
   next commit without having to modify the contract in the framework.

Change-Id: I2c8fab2c9b745b86e509500b09bd000f92474c52