Android: remove remaining explicit libcxx includes

Explicitly including libcxx includes is not necessary at least on
Android M and later. It appears that libc++ was made the default in
commit "Make libc++ the default STL." in Android build system post L.
However, if L support is still needed, using "LOCAL_CXX_STL=libc++" is
the preferred way.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>