adb: move win32 fd base to 2048, fix fd allocation.

Windows has a maximum fd limit of 2048, so we can avoid collision with
real file descriptors by starting from there. Also, fds would be
previously be allocated by a linear walk from the last allocated FD,
instead of the lowest available FD, as required by POSIX. Keep track of
the lowest available file descriptor to make things feel more familiar.

Change-Id: Id6ac1c54f4f7964a6cdfa8d3f4f96262e4881964