Reorganize char_utils.h and basechars.h

  * make BASE_CHARS[] const
  * add several inline menthods for ASCII character handling

Change-Id: I49664f219af88faf0aef43ac350cfc216570b185
diff --git a/native/Android.mk b/native/Android.mk
index f07be6a..d2537f0 100644
--- a/native/Android.mk
+++ b/native/Android.mk
@@ -12,6 +12,7 @@
     jni/com_android_inputmethod_keyboard_ProximityInfo.cpp \
     jni/com_android_inputmethod_latin_BinaryDictionary.cpp \
     jni/jni_common.cpp \
+    src/basechars.cpp \
     src/bigram_dictionary.cpp \
     src/char_utils.cpp \
     src/correction.cpp \