Use CPPFLAGS instead of CFLAGS.

While we build these as C, to the build system they are technically
C++ and are subject to the global CPPFLAGS. Set LOCAL_CPPFLAGS here
instead of LOCAL_CFLAGS so we can be sure we override anything
provided by the build system.

Bug: http://b/23043421
Change-Id: I344b54ae4ff9f142365a42c33ba160c1be17a342