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/ndk_headers.go b/cc/ndk_headers.go
index 4065128..b8423be 100644
--- a/cc/ndk_headers.go
+++ b/cc/ndk_headers.go
@@ -48,7 +48,7 @@
 }
 
 // Returns the NDK base include path for use with sdk_version current. Usable with -I.
-func getCurrentIncludePath(ctx android.ModuleContext) android.OutputPath {
+func getCurrentIncludePath(ctx android.ModuleContext) android.InstallPath {
 	return getNdkSysrootBase(ctx).Join(ctx, "usr/include")
 }
 
@@ -94,7 +94,7 @@
 }
 
 func getHeaderInstallDir(ctx android.ModuleContext, header android.Path, from string,
-	to string) android.OutputPath {
+	to string) android.InstallPath {
 	// Output path is the sysroot base + "usr/include" + to directory + directory component
 	// of the file without the leading from directory stripped.
 	//