Consolidate Bazel conversion state into a single field in CommonProperties

Bazel conversion state needs to be cloned during module cloning, so it
should be registered via module.AddProperties. Until now it was done
implicitly as the modules converted so far added the whole
base.commonProperties field (and thus transitively all its subfields).
However, adding CommonProperties also adds its fields to the list of
the module's attributes, which is undesirable. The problem surfaced
while implementing Bazel conversion to the `license` rule.

Bug: 190817312
Test: treehugger
Change-Id: Id3de4ede8df81b21f00065a3a1bdc2d707391c3a