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_sysroot.go b/cc/ndk_sysroot.go
index e39bae5..f6de4ef 100644
--- a/cc/ndk_sysroot.go
+++ b/cc/ndk_sysroot.go
@@ -66,12 +66,12 @@
 	pctx.Import("android/soong/android")
 }
 
-func getNdkInstallBase(ctx android.PathContext) android.OutputPath {
-	return android.PathForOutput(ctx, "ndk")
+func getNdkInstallBase(ctx android.PathContext) android.InstallPath {
+	return android.PathForNdkInstall(ctx)
 }
 
 // Returns the main install directory for the NDK sysroot. Usable with --sysroot.
-func getNdkSysrootBase(ctx android.PathContext) android.OutputPath {
+func getNdkSysrootBase(ctx android.PathContext) android.InstallPath {
 	return getNdkInstallBase(ctx).Join(ctx, "sysroot")
 }