Add background thread notes for package restriction serialisation.

Serialising package restrictions uses synchronous disk access; callers
of these methods should probably use background threads for this.

Bug: 149216360
Test: TreeHugger
Change-Id: I6607a7225bf7daaad8a78e4d1e4c585ba5ac3efc
Signed-off-by: Julius D'souza <jdsouza@google.com>