Merge "Revert "unique_fd: easier to forward declare"" am: 9d599c3ec6 am: 9d64cee049 am: de0d84ec35

Original change: https://android-review.googlesource.com/c/platform/system/libbase/+/1690407

Change-Id: I87ff394dac9ccb07e56d9408c14f1a63a83fddc5