Add timeout when waiting to bind to JobService

BUG: 17322886

bindService() to an invalid service might never actually result in
onServiceConnected being called , for e.g. if the client service doesn't
actually implement JobService. This wastes an execution slot as we end
up waiting forever.

Also made the javadocs clearer for the JobScheduler class.

Change-Id: Ie15ebbe18c0b7579f2ab77dd46428d354ef632c3