Use the correct mainline BCP jars for app dexpreopt.

Before this change, the build system had special logic to find mainline
BCP jars and copy them to a special location for app dexpreopt. This
logic doesn't work on `next`. In fact, we don't need this logic anymore.
Since we are now generating the mainline boot image extension, we can
reference the inputs of the mainline boot image extension, which are
exactly the mainline BCP jars needed for app dexpreopt.

Bug: 309302263
Test: atest art_standalone_dexpreopt_tests (on next)
Test: m --no-skip-soong-tests nothing

Change-Id: I055018ffbc5d7e5678f305e65a7f1c7e73bf3b99