Modify static lib stub library visiblity

java_sdk_library generates stub java_library module which compiles the
stub generated from either source or txt files, which is toggled based
on the build configuration. The java_api_library/java_library module
that compiles the stubs should not be directly accessible to the modules
outside of the sdk_library module scope, thus modify their visibility as
private.

Test: m nothing
Bug: 299373105
Change-Id: Ia4a08bf6e65c521814c82f54ea43cc8d367834e3