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

rust/rust.go

@@ -1046,9 +1046,9 @@ func (mod *Module) Name() string {
 	return name
 }
 
-func (mod *Module) setClippy(clippy bool) {
+func (mod *Module) disableClippy() {
 	if mod.clippy != nil {
-		mod.clippy.Properties.Clippy = proptools.BoolPtr(clippy)
+		mod.clippy.Properties.Clippy_lints = proptools.StringPtr("none")
 	}
 }