rust: modify linting properties

Move the linting properties to an enum with 4 possible options:
"default", "android", "vendor" or "none". The previous logic for
default, based on the module's location, is kept. It is now possible to
force the upgrade to a certain lint level for some modules (e.g.
external/[...]/android). Update the unit tests and documentation.

Bug: 163400111
Test: m
Change-Id: I8e464b04401158ed2d3c518a9b72f145a9835c99