Fixe race condition in AndroidTaskScheduler

This branch fixes a race condition in AndroidTaskScheduler when scheduling a task with a zero (or very small) delay. When adding a task to the queue, the scheduler schedules a check for tasks in the queue that are due to run. If this check happens before the new task has been added to the queue then the new task won't run until the next task is due to run or the periodic alarm fires, whichever happens first.

Closes #2306 (closed)