docs: Update permissions docs for Android M

Updates the docs to reflect the new Android M "runtime permissions"
model. (There's also a new training class, in a separate CL:
http://ag/762246 ) Removed the M Preview permissions page (but didn't
remove all references to it--Q will do that in a separate CL).

See first comment for doc stage location.

bug: 23725768

Change-Id: Iafb801f1188d1bb6b66ac19c9c1dfce0114df271