Separate InstallPath from OutputPath

Create a new type InstallPath that is similar to OutputPath to
differentiate intermediates output paths from installed output
paths.

RelPathString is a poorly defined, undocumented function that is
primarily used to get an install path relative to out/soong to
generate an equivalent install path for Make relative to $(OUT_DIR).
Move it to InstallPath for now, and fix the one remaining user on
OutputPath.

Add a method to create an NDK install path so that ndk_sysroot.go
doesn't have to do it manually with PathForOutput.

Bug: 141877526
Test: m checkbuild
Change-Id: I83c5a0bd1fd6c3dba8d3b6d20d039f64f353ddd5
diff --git a/android/prebuilt_etc.go b/android/prebuilt_etc.go
index d29ed16..3aeda1c 100644
--- a/android/prebuilt_etc.go
+++ b/android/prebuilt_etc.go
@@ -65,7 +65,7 @@
 	installDirBase string
 	// The base install location when soc_specific property is set to true, e.g. "firmware" for prebuilt_firmware.
 	socInstallDirBase      string
-	installDirPath         OutputPath
+	installDirPath         InstallPath
 	additionalDependencies *Paths
 }
 
@@ -91,7 +91,7 @@
 	return PathForModuleSrc(ctx, String(p.properties.Src))
 }
 
-func (p *PrebuiltEtc) InstallDirPath() OutputPath {
+func (p *PrebuiltEtc) InstallDirPath() InstallPath {
 	return p.installDirPath
 }