apex: Don't run apex mutators if disabled

apex_deps/apex/apex_flattened mutators don't have to run if module is
disabled.

For some branches which have no valid targets, apex modules are disabled
by the os mutator. e.g. aosp-build-tools

Some enforcement checks which run during those mutators may fail because
build-environment doesn't provide full context information.

For example, "Platform_version_all_codenames" config variable is not
set while apex.min_sdk_version uses one of those codenames.

Bug: 152655956
Test: m
Change-Id: I47e27d2d025ba4c36534708b113ce77c4cb2397b