docs: New permissions training class.

Describes how to declare that your app needs system permissions;
also describes how (on M and later) to request those permissions at
runtime.

There's a companion CL which updates various existing API docs for
the new M permissions model (http://ag/762405 ).

See first comment for doc stage location.

bug: 23725768

Change-Id: I7c3babdb9348969d1b0b019e7e9ef7116b04c658