hiddenapi: Refactor before new features are added

Refactor the hiddenapi build tool to take a command name as its
first argument and create a notion of a "class path" group of dex
files. These changes will be needed for creating a new command
that resolves SDK stubs against the boot class path.

Bug: 79409988
Test: m hiddenapi
Test: m test-art-host-gtest-hiddenapi_test
Test: art/test.py -r -t 674-hiddenapi -t 999-redefine-hiddenapi
Change-Id: I93c36154cc8c5e8c0e9414cf02e2c6ea298ae0e8