Use java_sdk_library in bootclasspath_fragment contents as stubs

A java_sdk_library specified in the bootclasspath_fragment contents
must be providing APIs for the bootclasspath_fragment and so must be
treated as if they were specified in the stub_libs. This avoids having
to specify them in both contents and stub_libs.

Bug: 179354495
Test: m nothing
Change-Id: I535065ee1a79b439e2676f35e06a75d4626adcaf