Adds a new ACCESS_CONTEXT_HUB permission

Since not all Context Hub applications are location-related, the
use of the LOCATION_HARDWARE permission to control access to the Context Hub
System APIs can be misleading. Create a new permission to access it, and
adds the logic to enforce the permission.

Also clarifies usage of PendingIntent ContextHubClient.

Bug: 135951924
Test: Compile and flash
Change-Id: I5f22db7e57ef4a6c3718acb38c4f313d6a04839f