Fix issue #22531747: Assist info should declare if user has disabled...

...context and/or screenshot

Added new API to find out what contextual data has been globally disabled.

Also updated various documentation to make it clear what kind of contextual
data you will get (and when it will be null).

Also added a new Activity.showAssist() API because...  well, I was already
in there, it was easy to do, it is safe, and maybe people will build cool
things with it.

Change-Id: Ia553d6bcdd098dc0fce4b9237fbfaca9652fc74b