Symlink file_contexts --> file_contexts.bin

The presence of /file_contexts is still important to TWRP:
- files written while in adb shell only pick up contexts if
  /file_contexts is present
- fixContexts looks for /file_contexts
- if !USE_EXT4 (is this possible), then make_ext4fs looks for
  /file_contexts when wiping

In the event than file_contexts.bin also becomes more important in the
future, leave it in place and create a symlink to it.

Change-Id: Ic87852248d42d5ea6bf936df160efa41294b6520