adb: more immediately try to reconnect connections.

Most disconnects we're likely to encounter are cases where either we
notice immediately and can start reconnecting almost immediately (adbd
restarting because of `adb root`, etc.), or where we won't notice for a
while anyway, so a 10 second sleep is somewhat meaningless.

Test: adb root; time adb wait-for-device shell
Change-Id: I18e9213dc4e84d735e9240118a368dcb38f21c78