Support partial update for prebuilts

Bug: 203231943
Test: prebuilts/runtime/mainline/update.py -b aosp-master --build $NUM \
        --install-module platform
      # Saw only changes under platform
Test: prebuilts/runtime/mainline/update.py -b aosp-master --build $NUM
      # Still saw changes under all directories. Commit message also
      # looks correct.
Test: prebuilts/runtime/update.py -b aosp-master --build $NUM
      # Still saw changes under all directories

Change-Id: Iab33e8efad6c08ec74694572cd3d39cf79cd0bad
4 files changed