Move legacy obb data migration to a handler thread.

The operation can potentially take a long time to complete
depending on the volume of data to be copied, so move it off
the ActivityManager handler thread that needs to be available
for other operations.

Bug: 134570017
Test: manual; set a 1 minute sleep in migrate_legacy_obb_data.sh

Change-Id: I3d2c52e8b012ed71c53810e6919d11be9a97cc6c