Drop "prebuilt_" prefix when adding libraries to class loader context.

This is needed when some source libraries get replaced with prebuilt
variants: CLC should contain one entry for a library (the prebuilt one
or the source one, whichever of them is preferred). Because the prebuilt
module name starts with "prebuilt_" prefix, previously Soong considered
such libraries as two different ones libraries and added both to CLC.

Bug: 193425964

Test: mark "prebuilt_android.net.ipsec.ike" as preferred and `m nothing`
      (before this CL it would fail the build, now it builds fine).

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' -C 1
      # empty output, no errors
Change-Id: Icc42533d9915060d7fffda12aa93b9d18dc4f83d