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/cc/installer.go b/cc/installer.go
index a52ccf1..610252c 100644
--- a/cc/installer.go
+++ b/cc/installer.go
@@ -52,7 +52,7 @@
 	relative string
 	location installLocation
 
-	path android.OutputPath
+	path android.InstallPath
 }
 
 var _ installer = (*baseInstaller)(nil)
@@ -61,7 +61,7 @@
 	return []interface{}{&installer.Properties}
 }
 
-func (installer *baseInstaller) installDir(ctx ModuleContext) android.OutputPath {
+func (installer *baseInstaller) installDir(ctx ModuleContext) android.InstallPath {
 	dir := installer.dir
 	if ctx.toolchain().Is64Bit() && installer.dir64 != "" {
 		dir = installer.dir64