Remove restriction on exported plugins that generate APIs

hilt_android requires seven separate annotation processors, which
is only feasible to support using exported_plugins to avoid having
to list all seven in every module that uses it.  Unfortunately they
all set generates_api: true.  Turbine is already disabled for modules
that directly use a plugin that sets generates_api: true, because
turbine doesn't run annotation processors.  Also add support for
disabling turbine if a module transitively uses a plugin that
generates APIs via exported_plugins.

Bug: 173397767
Test: TestExportedPlugins
Change-Id: If70354a3dd67efb4ce88bc9c934d41ccb6241b28