Add null check to cleanupRecentTasksLocked.

If during boot, the package manager broadcast occurs before
TaskPersister has restored the recents list then mRecentTasks will be
null and an NPE will occur. This checks for null mRecentsTasks and
returns before the NPE happens.

Fixes bug 17300409.

Change-Id: I1588ad3a3c0d04bc6f273630ef5dadf6821e5710