Fire timers as scheduled after app upgrade

Bug: 26676588

Timer refactoring has removed TimerReceiver, however, timers
scheduled in 4.2.1 and prior use TimerReceiver as the entry point
for expiring a timer. TimerReceiver must be kept in place for at
least one release cycle in order to correctly fire legacy timers at
their scheduled time.

Change-Id: I0e3a751fce981fc22911d869f375b0cd40dde438