Merge "apex: add / in file_contexts as system_file" am: fae9e0cf9a am: 062e9afe7e am: ab22ca31e9

Original change: https://android-review.googlesource.com/c/platform/build/soong/+/1351465

Change-Id: I7f85dd364c3dae5b8151ba984ab71d09c2f8229a