local-source-file-path does not expand to correct file paths in
some circumstances so some users override it.  Therefore
flatbuffers_header_build_rules has been modified to generate rules
that expand LOCAL_SRC_FILES values with flatbuffers_header_build_rules.

Also, this overrides local-source-file-path to allow nest projects
to build when NDK_OUT is set.

Tested:
Verified a dependent project continues to build.

Bug: 25673744
Change-Id: Ic90186fe96d6e4533f9f3b7ca9ef78084de08a7e