Make the default java_sdk_library dist_group "unknown"

Change the default dist_group from "android" to "unknown" to prevent
accidentally including java_sdk_library stubs that do not set
dist_group or owner in the public SDK.

Bug: 186723288
Test: TestJavaSdkLibraryDist
Change-Id: I9aae2a16254ac1a8d444acfa63bc571d1ef4b045
Merged-In: I9aae2a16254ac1a8d444acfa63bc571d1ef4b045
(cherry picked from commit 3dd66250)