Do not hold wm lock when loading recent task files

The task files are now loaded into ByteArrayInputStream without
any lock. Then the existing xml parser can still accept it as
InputStream to restore task from byte array directly in wm lock
without accessing storage.

PackageConfigPersister uses its own lock, so it doesn't need to
be in wm lock.

Fix: 323134961
Test: atest RecentTasksTest TaskPersisterTest

Change-Id: I0899e8835b21e30deebd024700e345759081b209