Use LOCAL_ADDITIONAL_M4DEFS for file_contexts

Pass LOCAL_ADDITIONAL_M4DEFS to m4 when building vendor_file_contexts
and odm_file_contexts. The build command attempts to use
PRIVATE_ADDITIONAL_M4DEFS - but this is not set in the target-specific
variables.

This allows using custom M4 macros when building non-platform
file_contexts.

Change-Id: I5fa8d9ec91f1a97bee1dd735ba85af93eef91252