java_sdk_library - replace .docs suffix with .stubs.source

Previously, the .docs suffix was used for droidstubs internal modules
that generate the stubs source. The name was confusing so this change
replaces the suffix with .stubs.source to clarify the purpose of those
modules.

Bug: 148080325
Test: m droid
Change-Id: I9d7a2ab19e4a331b515408912ac8f7e8b0e1b35d