Don't attempt to add stub libraries to class loader context.

A Java module may depend on a stub library. In that case an additional
dependency on the implementation library is created, and it is used to
add the implementation library to class loader context. We should not
attempt to add the stubs library as well (previously the attempt to add
it happend after the implemention was added to CLC, to the attempt was
unsuccessful).

Raise an error if someone tries to add the same library with different
build/instal paths.

Also, rename local variable `implicitSdkLib` to `sdkLib` to better
reflect its meaning.

Bug: 193425964

Test: $ lunch aosp_cf_x86_64_phone-userdebug && m && launch_cvd
      $ adb wait-for-device && \
        adb root && \
        adb logcat | \
        grep -E 'ClassLoaderContext [a-z ]+ mismatch' -C1
      # empty output, no errors
Change-Id: I01c1bdd23f9d118d891d0b806e7e3b4d78896a34