Support building libjnigraphics on the host

Bug: 169137236
Test: SANITIZE_HOST=address make imagedecoder_fuzzer

Along with a change in frameworks/base
(I0ffc4e90b4083db79fbb12012e2e1284206b43e3), this will make it possible
to run imagedecoder_fuzzer on the host, for faster fuzzing.

imagedecoder.h:
- define __ANDROID_API__ and __INTRODUCED_IN on non-Android systems, so
  methods can be called on those systems.

bitmap.h:
- define __ANDROID_API__ and __INTRODUCED_IN on non-Android systems, so
  methods can be called on those systems.
- include <stddef.h>, which is needed on non-Android systems for size_t.
- on Android, unguard types and enums for easier opportunistic usage via
  dlsym.

nativewindow/Android.bp
- make libnativewindow_headers host_supported, so that libjnigraphics
  can access android/data_space.h

Change-Id: Iefdb4132d6f10f9164be85f4dc6e11883f6fd37e