build: Move zlib flag to preferred file.

Apparently, the best way for a static lib to let its clients know what
system libraries they should link with is to use LOCAL_EXPORT_LD_LIBS
flag.

From the NDK 8d docs:

LOCAL_EXPORT_CFLAGS
  Define this variable to record a set of C/C++ compiler flags that will
  be added to the LOCAL_CFLAGS definition of any other module that uses
  this one with LOCAL_STATIC_LIBRARIES or LOCAL_SHARED_LIBRARIES.

*** snip ***

LOCAL_EXPORT_LDLIBS
  Same as LOCAL_EXPORT_CFLAGS, but for linker flags. Note that the
  imported linker flags will be appended to your module's LOCAL_LDLIBS
  though, due to the way Unix linkers work.

  This is typically useful when module 'foo' is a static library and has
  code that depends on a system library. LOCAL_EXPORT_LDLIBS can then be
  used to export the dependency. For example:

      include $(CLEAR_VARS)
      LOCAL_MODULE := foo
      LOCAL_SRC_FILES := foo/foo.c
      LOCAL_EXPORT_LDLIBS := -llog
      include $(BUILD_STATIC_LIBRARY)

      include $(CLEAR_VARS)
      LOCAL_MODULE := bar
      LOCAL_SRC_FILES := bar.c
      LOCAL_STATIC_LIBRARIES := foo
      include $(BUILD_SHARED_LIBRARY)

  There, libbar.so will be built with a -llog at the end of the linker
  command to indicate that it depends on the system logging library,
  because it depends on 'foo'.