(1) Use ALL_CAPS name for the environment variable with the C/C++ compilation

flags: MY_LIBTEXTCLASSIFIER_CFLAGS According to
https://developer.android.com/ndk/guides/android_mk.html#var, lower-case names
are reserved.  From there: "If you need to define your own convenience variables
in an Android.mk file, we recommend prepending MY_ to their names."

(2) Use env variable $(LIBTEXTCLASSIFIER_STRIP_OPTS) and document it.  One can
use e.g.,

mmma -j16 LIBTEXTCLASSIFIER_STRIP_OPTS=keep_symbols external/libtextclassifier

to build with all symbols in, such that treemap output is meaningful.  Then,
inside Google, one can run

/home/build/google3/geo/render/mirth/tools/binary_size_treemap.sh \
  -b /usr/local/google/home/salcianu/android2/master/out/target/product/bullhead/obj_arm/lib/libtextclassifier.so \
  -o /tmp/bar

and next open file:///tmp/bar/index.html in a browser.

(3) Rearrange a bit some of the lines.  E.g.,

LOCAL_PATH := $(call my-dir)

should be at the top of the file, not in the section for a specific module, so I
moved it before the comment for the first module.

Change-Id: I7c705b19ef29e1900b68f6bdf3a7d58d8ee17cc3

Test: build with symbols and then run treemap (see point 2 above)