Rename .c files compiled as c++ to .cpp

Compiling .c files as .cpp with -x c++ -std=c++11 is delicate, rename
the .c files to .cpp instead.

Also fix some other makefile issues:
Use LOCAL_EXPORT_C_INCLUDE_DIRS and LOCAL_EXPORT_SHARED_LIBRARY_HEADERS
instead of $(call include-path-for,...), which is deprecated.
Remove LOCAL_MODULE_TAGS := optional, that is the default.

Test: mma -j
Change-Id: I482be683abb4dffbb46fb4d1af07a5ec8fcfb908