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/rust/compiler.go b/rust/compiler.go
index 3bfef76..76d5ad8 100644
--- a/rust/compiler.go
+++ b/rust/compiler.go
@@ -94,7 +94,7 @@
 	dir64    string
 	subDir   string
 	relative string
-	path     android.OutputPath
+	path     android.InstallPath
 }
 
 var _ compiler = (*baseCompiler)(nil)
@@ -173,7 +173,7 @@
 	return compiler.Properties.Crate_name
 }
 
-func (compiler *baseCompiler) installDir(ctx ModuleContext) android.OutputPath {
+func (compiler *baseCompiler) installDir(ctx ModuleContext) android.InstallPath {
 	dir := compiler.dir
 	if ctx.toolchain().Is64Bit() && compiler.dir64 != "" {
 		dir = compiler.dir64