Correctly install APK-in-APEX

APK in a flattened APEX is installed as 'ETC' class module, instead of
the 'APP' class. This is to prevent Make from doing app-specific
amendments (e.g. such as adding module name after my_module_path)
to the paths and filenames which are all correctly set in the Soong
side.

Test: add `installable:true` to development/samples/Snake/Android.mk
and add 'Snake' to the `apps` property of an APEX. The build is
successful.
Test:  build the APEX on a device with TARGET_FLATTEN_APEX=true
The APK is at system/apex/<apexname>/app/Snake/Snake.apk

Bug: 142537672

Change-Id: I314bf1ab7abd4d4a4e9fa210442c004f54d8ccca

apex/apex.go

@@ -558,7 +558,14 @@ func (class apexFileClass) NameInMake() string {
 	case nativeTest:
 		return "NATIVE_TESTS"
 	case app:
-		return "APPS"
+		// b/142537672 Why isn't this APP? We want to have full control over
+		// the paths and file names of the apk file under the flattend APEX.
+		// If this is set to APP, then the paths and file names are modified
+		// by the Make build system. For example, it is installed to
+		// /system/apex/<apexname>/app/<Appname>/<apexname>.<Appname>/ instead of
+		// /system/apex/<apexname>/app/<Appname> because the build system automatically
+		// appends module name (which is <apexname>.<Appname> to the path.
+		return "ETC"
 	default:
 		panic(fmt.Errorf("unknown class %d", class))
 	}