Fix sharded-jarjar output path

Make sure to use a relative path.

Flag: EXEMPT trivial bugfix
Bug: 383559945
Test: $ANDROID_BUILD_TOP/development/tools/ndk/update_ndk_abi.sh
    # only make sure ninja-gen works

Test: OUT_DIR=$ANDROID_BUILD_TOP/out m nothing --no-skip-soong-tests

Test: $ANDROID_BUILD_TOP/frameworks/base/ravenwood/scripts/run-ravenwood-tests.sh -s

Test: treehugger
Change-Id: I3f2c097f9a8a047133e9bc246df38549b922507a