Add buildDir to WritablePath implementations

First, the buildDir() method was renamed to getBuildDir() to avoid
clashing with the buildDir field.

Then, a buildDir was added to both `OutputPath` and `InstallPath` but
not to `PhonyPath` as it does not contain any path components. Instead
the `PhonyPath.getBuildDir()` was changed to simply return "".

Bug: 183650682
Test: m droid
Change-Id: I12e1854c829b980c5c01205753c62c00dc0a4774

android/testing.go

@@ -909,7 +909,7 @@ func NormalizePathForTesting(path Path) string {
 	}
 	p := path.String()
 	if w, ok := path.(WritablePath); ok {
-		rel, err := filepath.Rel(w.buildDir(), p)
+		rel, err := filepath.Rel(w.getBuildDir(), p)
 		if err != nil {
 			panic(err)
 		}
@@ -944,7 +944,7 @@ func PathRelativeToTop(path Path) string {
 	}
 	p := path.String()
 	if w, ok := path.(WritablePath); ok {
-		buildDir := w.buildDir()
+		buildDir := w.getBuildDir()
 		return StringPathRelativeToTop(buildDir, p)
 	}
 	return p