contexthub: Expose nanoapp enabled/disabled state

Add a flag to HubAppInfo that identifies whether an app is enabled and
running, or loaded but disabled. Also, make comments for structure
fields consistently start with a capital letter, and do some minor
comment cleanup.

Test: compile test only, adding a new field to an existing structure
Change-Id: Ia033afd3903d24e45235c70da21681669a53f4bf