adb: extract the adb-specific unique_fd out into its own header.

Some POSIX-only files would like to use functions declared in
adb_utils.h without being forced to use the sysdeps stuff for open,
close, etc. Extract the adb-specific unique_fd into its own header so
those files can use adb_utils.h alongside the real unique_fd.

Change-Id: I10344ef26b7e72fce46bf7e3033f89aa54bd6118
Test: mm