Rename Looper::isIdling() to isPolling() to resolve confusion.

The loop isn't technically idle at this time, it's just checking
whether any file descriptors have pending events.  However it's
still a good signal as to whether the loop is alive.

Added a real isIdle() function.

Bug: 19532373
Change-Id: Idd273e8774f469ccafb00d560818cf279dfd6ba6