Remove empty directories when removing previously installed files

Apparently PackageManager gets confused when the apk disappears, but the
directory still exists.

Test: lunch aosp_arm-eng; m BasicDreams
      <remove BasicDreams from PRODUCT_PACKAGES>
      m nothing
      ls out/target/product/generic/system/app
Change-Id: I486c0ddadde55fad226049d3fb0f09af88117da0