Add OS to configuration key in mixed builds

This also removes the special-case filegroup from mixed builds
buildroot; no special handling is required. Since we're currently
hardcoding linux_x86_64 as our host platform, it should be fine
to continue assumping that hardcoded host for now.

Test: USE_BAZEL_ANALYSIS=1 m adbd
Change-Id: I35509f4eb33ba7a243fab4c34b35958f6f2fceab
diff --git a/android/filegroup.go b/android/filegroup.go
index f8f0955..a79374d 100644
--- a/android/filegroup.go
+++ b/android/filegroup.go
@@ -118,14 +118,16 @@
 	}
 
 	archVariant := ctx.Arch().ArchType
+	osVariant := ctx.Os()
 	if len(fg.Srcs()) == 1 && fg.Srcs()[0].Base() == fg.Name() {
 		// This will be a regular file target, not filegroup, in Bazel.
 		// See FilegroupBp2Build for more information.
 		archVariant = Common
+		osVariant = CommonOS
 	}
 
 	bazelCtx := ctx.Config().BazelContext
-	filePaths, ok := bazelCtx.GetOutputFiles(fg.GetBazelLabel(ctx, fg), archVariant)
+	filePaths, ok := bazelCtx.GetOutputFiles(fg.GetBazelLabel(ctx, fg), configKey{archVariant, osVariant})
 	if !ok {
 		return
 	}