Move copying of dex files from dexpreopt_bootjars singleton

The art dex files are copied in the bootclasspath_fragment and the
non-updatable and updatable dex files are copied in the
platform_bootclasspath.

Bug: 177892522
Test: m nothing
Change-Id: I5d3d533d1a7a9f8e7ae20c12eb33029a898a2cd6