Fixed subtle bugs related to file descriptor recycling.

Added code to protect against situations that may occur when a
Looper callback has the side-effect of closing the file descriptor that
it is watching before it returns.  This code pattern is very
convenient for implementation but it does expose issues in how
the list of callbacks is maintained.  In particular, we
need to watch out for file descriptors which have been reused.

This change may resolve previously unexplained ANRs associated with
log messages such as: "Error modifying epoll events for fd 44, errno=2"

Bug: 10349083
Change-Id: I20eedf6ffbdeda382653ca0104962505194741b0