Allocate OutputPath.String in PathForOutput

In an AOSP aosp_blueline-userdebub build, OutputPath.String was
allocating 802MB of strings in filepath.Join to prepend the
out dir to each path.  Allocate the joined string in PathForOutput
instead, which results in ~57MB of new allocations in PathForOutput
but no allocations in OutputPath.String.

Test: all soong tests
Change-Id: Id452e0c46a2aeda71bfac11a227bb6edb8e3523d