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/java/platform_compat_config.go b/java/platform_compat_config.go
index 3d46077..0c973c5 100644
--- a/java/platform_compat_config.go
+++ b/java/platform_compat_config.go
@@ -30,7 +30,7 @@
 	android.ModuleBase
 
 	properties     platformCompatConfigProperties
-	installDirPath android.OutputPath
+	installDirPath android.InstallPath
 	configFile     android.OutputPath
 }