Fix libtextclassifier makefile to be reproducible

Use standard macros instead of handcoded $(shell find) calls that don't
return reliably ordered results.

Also clean up some other makefile bits while I'm here:
* LOCAL_MODULE_TAGS := optional is the default
* Version scripts need to be depended on with
  LOCAL_ADDITIONAL_DEPENDENCIES.
* There's no need to include . in LOCAL_C_INCLUDES (and that's the
  top of the source tree)

Bug: 36463318
Test: mmma -j external/libtextclassifier
Test: ensure LOCAL_SRC_FILES / LDFLAGS is the same before/after this
change (minus sorting)
Change-Id: I3b0ab1bf9b42d1db2e865ff5c07d29868b94601c