java_sdk_library: Rename createDocs to createStubsSources

The createDocs(...) method was obviously named because it created a
droiddocs target that generated the stubs source but it now creates a
droidstubs target so the name was misleading.

Bug: 145998881
Test: m checkbuild
Change-Id: I7419b0a01ee87ecb2b396e4817e5e88a88a8b7b6