Add IsAddingDependency to IncomingTransitionContext

Add an IsAddingDependency method to IncomingTransitionContext that
returns true if IncomingTransition is called after the transition
has already won while adding a new dependency.  This will be used
as part of the transition mutators to support an apex use case where
incoming dependencies during the initial apex mutator need to be
rewritten onto the platform variant for modules that don't support
the apex, but a later call to OtherModuleDependencyVariantExists
must not rewrite the requested apex variation onto the platform
variant.

This should be used sparingly, all uses will have to be removed in
order to support creating variants on demand.

Bug: 319288033
Test: TestIsAddingDependency
Flag: EXEMPT refactor
Change-Id: Ib8e419d35ff8f7cbff9667c1cd40d05ccfacab8b