Resolving race condition while writing recent taskids

There was a race condition, due to which, by the time
TaskPersister started writing the taskids to the file,
the user was stopped and its data from memory was unloaded,
resulting in a NullPointerException

Bug: 30944155
Change-Id: Iac3333b7744241c90a7769686983e3f16e6880c1