Install required deps for flattened APEX.

The primary boot image is a required dep of the ART APEX. It needs to be
installed no matter the APEX is flattened or not.

Bug: 220991677
Test: 1. m OVERRIDE_TARGET_FLATTEN_APEX=true com.android.art
  2. See boot.art in /system/framework/<isa>
Change-Id: I1a69ae1b86df97fc44ea6c75c8bb9ec960c8d249