adb: keep all asocket operations in the main thread.

As far as I can see, all asockets operations happen in fdevent_loop()
in the main thread, excepting close_all_sockets(). Instead of adding
lock and ref_count for each asocket, a simpler way would be moving
close_all_sockets() from input_thread to the main thread.

In input_thread(), there are two path to break the loop and call
close_all_sockets(). One path is when receiving offline A_SYNC, which
is sent by the main thread. The other path is when read_packet
fails, which I believe is almost not possible and doesn't matter
(Because t->fd is closed just before t is freed.). So I move
close_all_sockets() to handle_offline() in the main thread.

the socket_list_lock in sockets.cpp could be removed. But I prefer
to leave it for the following changes.

Bug: 6558362
Change-Id: I5da23f60a67a331262c62693b9b127fe2689c799