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
70dda7e3da