Grant the shell user a permission to let apps host widgets.

An app can host widgets only if it has a special signature premission
or the user allowed this app to be able to bind to widgets. This change
grants the permission to whitelist an app to bind to app widgets to the
shell user. Now a test can have a host that will have some widgets. Note
that this is different from whitelisting corp widgets.

bug:17993121

Change-Id: Ic6b866cb2a822254e94a130b876ee5db66ed4fbf