Use correct install paths in generated Android.mk

Extract Soong's install path and put it in the generated Android.mk file
so that tests get installed in the correct place.

Change-Id: Id4726855c5677855406de20773a5da533bdd4cea
diff --git a/common/paths.go b/common/paths.go
index 93cad13..554ce3f 100644
--- a/common/paths.go
+++ b/common/paths.go
@@ -478,6 +478,10 @@
 	return filepath.Join(p.config.buildDir, p.path)
 }
 
+func (p OutputPath) RelPathString() string {
+	return p.path
+}
+
 // Join creates a new OutputPath with paths... joined with the current path. The
 // provided paths... may not use '..' to escape from the current path.
 func (p OutputPath) Join(ctx PathContext, paths ...string) OutputPath {