Optimize FdEventsReader to make it safer to use.

Update the start() and createAndRegisterFd() to boolean type, to make
sure we won't return null socket when failing to create socket.

Throwing IllegalStateException if any caller is calling the FdEventsReader#
start/stop from the different thread, to make it immediately obvious.

Bug: None
Test: atest NetworkStackTests
Change-Id: I398b24da34b8fc9da7e8a9d07508c4d731c4a437