[zircon/ftl] Changed FTL-NDM interface from callbacks to direct function calls.

1. Because Google app doesn't require callback interface between the FTL and
   the NDM, removed it to increase clarity and decrease lines of code.
2. Changed some local variables from 'ftl' to 'ftl_cfg' to better differentiate
   between control block handles and temporary configuration structures.
3. Removed ENOMEM definition from 'kernel.h'.
4. Removed FsError() calls from routines in 'fsmem.c' since their callers
   invoke FsError2() if there is an error.
5. Introduced 'ftl_pri.h' (for internal use) and 'ftl_pub.h' (for driver).
   Following check-ins will consolidate other headers into those files.

Change-Id: I6c27dfd24755c5fda1e0cb5ba09e8707ce31b653