Remove support for droidstubs in sdk/module_exports

The droidstubs support in sdk/module_exports was a temporary measure
to work around the fact that some APIs were being defined by direct use
of droidstubs instead of java_sdk_library.

This change removes that support as those APIs have all been switched
from droidstubs to use java_sdk_library so droidstubs support is no
longer needed.

Bug: 168301990
Test: m nothing
Change-Id: I3517bed29b030438a0423a6cb8c248992a988222