Simplify arch target handling

Soong's multi-architecture building has grown complex, with the
combination of HostOrDevice+HostType+Arch necessary to determine how to
build a variant of a module, and three separate mutators to split each
into its variations.

Combine HostOrDevice+HostType into Os, which will be Linux, Darwin,
Windows, or Android.  Store Os+Arch as a single Target.

Change-Id: I92f2e2dac53617d595a35cc285d2bd348baa0fbd

android/paths.go

@@ -629,7 +629,7 @@ func PathForModuleInstall(ctx ModuleContext, paths ...string) OutputPath {
 		}
 		outPaths = []string{"target", "product", ctx.AConfig().DeviceName(), partition}
 	} else {
-		outPaths = []string{"host", ctx.HostType().String() + "-x86"}
+		outPaths = []string{"host", ctx.Os().String() + "-x86"}
 	}
 	if ctx.Debug() {
 		outPaths = append([]string{"debug"}, outPaths...)