Improved build rule generation for Android flatbuffer headers.

* Added the ability to create a build target for generated headers.
* Made it possible for generated header targets to depend upon each
  other or arbitrary build targets.

Tested:
Verified some pretty complex libraries with numerous flatbuffer schema
dependencies build using this macro on Linux with the NDK.
Bug: 25188384

Change-Id: I846855a50808e58c34cdf7086e93e7ea0df69e0d