cp preopted files from B partition during PM init.

This allows us to save space on the system partition while still
having access to the preopted files.

We do this on first boot when the "ro.cp_system_other_odex" property
is set to 1. We do this during package manager initialization before
scanning the system to see which apks need to be optimized again.

Note that a separate script, run by init, is actually responsible for
finding and copying the files. We simply request that it runs.

Bug: 29278988

Change-Id: I8d7c790ad35b32a0ce1d87939f043419bae4d88a
(cherry picked from commit 3dafee6c)