Fix NdkImage.h includes

It needs stdint.h.

It's OK to include android/hardware_buffer.h on pre-26. We'd like to remove
the __ANDROID_API__ guard around AImage_getHardwareBuffer, and if/when we
do, we'll need android/hardware_buffer.h. The last time we removed the
__ANDROID_API__ guards around the APIs, we forgot to remove the one around
this #include.

Bug: http://b/111130072#comment12
Test: builds
Change-Id: Ice8073476d4ad4802e3540f15eee3af29e0b891f