Support for dexopt postprocessing in merge_target_files.

When using the VSDK, dexopt is not applied during the vendor build.
To avoid a first-boot time regression, dexopt is applied during the
merge stage, by running dexopt on the vendor apps and rebuilding
the vendor image.

Bug: 188179859
Test: Tested in keystone with VSDK target
Change-Id: Ie8e2d0a82850a2901fa6f250433bcbb43f0a97f2