Bp2build: handle the case where java_library has only proto srcs

java_library doesn't accept deps when there are no srcs because
no compilation happens, but it accepts exports.

The deps from the module are not necessary for compiling the protos,
in which case they are unnecessary as deps on the java_library as well
since they are not be propagated to any dependencies.

So we can put the deps to exports and drop deps here.

Test: CI and added unit test

Bug: 285952385
Change-Id: Ie54a4ac0db592fb96fede64f0e67df309dca9c1d