Fix contexts modules to use `android:"path"`

For now, contexts modules have been using se_filegroup modules, which
makes the build system logic unnecessarily complex. This change
refactors it to se_build_files modules and normal `android:"path"`
logic.

Test: build and boot
Change-Id: I52e557e2dc8300186869a97fddfd3a74183473f7
diff --git a/apex/Android.bp b/apex/Android.bp
index 5276cca..166c2d3 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -22,6 +22,11 @@
 }
 
 filegroup {
+  name: "apex_file_contexts_files",
+  srcs: ["*-file_contexts"],
+}
+
+filegroup {
   name: "apex.test-file_contexts",
   srcs: [
     "apex.test-file_contexts",