Adding shell commands for grant/revoke of bind app widget permisison.

Testing the app widget APIs requires binding of widgets which is guarded
by a system signature permission or white listing. Since CTS tests can
not be signed with the platform certificate, they should be able to
temporariliy white list their package. This command adds the ability to
grant and revoke the provilege to bind app widgets to a given package.

Change-Id: I6de1bbf27ac684ec47e61157f19ec6d29e2db979