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/android/notices.go b/android/notices.go
index 7b61d65..bf273b5 100644
--- a/android/notices.go
+++ b/android/notices.go
@@ -60,7 +60,7 @@
 	})
 }
 
-func BuildNoticeOutput(ctx ModuleContext, installPath OutputPath, installFilename string,
+func BuildNoticeOutput(ctx ModuleContext, installPath InstallPath, installFilename string,
 	noticePaths []Path) NoticeOutputs {
 	// Merge all NOTICE files into one.
 	// TODO(jungjw): We should just produce a well-formatted NOTICE.html file in a single pass.