SocketListener: use poll() instead of select()

FD_SET is limited to 1024 file descriptors in Linux, which causes
processes with too many open files or connections to crash:

  FORTIFY: FD_ISSET: file descriptor 1024 >= FD_SETSIZE 128

The fix we used elsewhere is replacing select() with poll(), but in the
case of SocketListener we additionally need to replace the SocketClient
list with a map indexed by fd in order to avoid quadratic behavior on
each poll() wakeup.

Bug: 79838856
Test: device boots and appears to work normally, tests pass
Change-Id: I4a8f1804fa990d3db3a2c96b9acd60b2c7135950