Disable all-assigned check in hiddenapi on master-art

Master-art configurations do not have frameworks/base and therefore do
not have hidden API flags. Pass --no-force-assign-all to `hiddenapi`
when frameworks/base does not exist to disable the corresponding
assertion. This enables us to enforce the assertion on non-master-art
builds and also get rid of logspam about missing flags on ART buildbots.

Test: art/tools/buildbot-build.sh on master-art
Bug: 123143676
Change-Id: I074d9554fb11dab3eef904016375730520107ec2