Fix handling of "prebuilt_" prefix in classLoaderContextForUsesLibDeps.

Changes:
- Cut off "prebuilt_" prefix from `dep`, not `libName`, because `dep` is
  used to replace library name in `usesLibraryProperties`, and these
  properties are specified without "prebuilt_" prefix.

- Do not remove "prebuilt_" prefix on the return value of
  `ProvidesUsesLib()`: this boils down to `provides_uses_lib` property
  which is specified without the prefix.

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: I6cb65040027023d12ca0bc132d60c2c245174c40