libsuspend: move to exponential backoff

If for some reason the system can't suspend (usually a driver bug),
libsuspend will currently attempt to retry suspend after 100ms. Because
entering suspend takes a significant amount of CPU time, this can be
extremely expensive and be a major contributor to rapid battery
drain. Move autosuspend to use exponential backoff if the previous
suspend attempt failed.

bug 32092914

Change-Id: I3e9e944f290de5f1853a02e3f61721ba9159df46