Apply non-FBE migration early during boot.

On non-FBE devices, we offer to migrate CE/DE storage of certain
system apps during boot.  However, if this migration happens after
app code has started to run, we end up clobbering open files such
as databases.

The right way to solve this is to apply all migration early during
boot (in the PMS constructor) before any app code can start.

Bug: 28512897, 28525109
Change-Id: I17eb9a09e5862d873b86c05ce35874a34386e433