Stop making the ndk headers host-specific

This breaks when I enable a second host configuration for host bionic.

Make non-arch modules !Device and !Host by adding "Generic" as the zero
value. Make the SkipDeviceInstall config only apply to Device modules.

Test: out/soong/build.ninja is identical expect for comments
Change-Id: I6e2bd3814a9968554d3901a508d822d5c79d08a0

android/arch.go

@@ -217,7 +217,8 @@ type OsType struct {
 type OsClass int
 
 const (
-	Device OsClass = iota
+	Generic OsClass = iota
+	Device
 	Host
 	HostCross
 )