Include symlinks in flattened apexs

Flattened apexs were not including symlinks. This could cause expected
files not to be present.

Test: lunch aosp_marlin-userdebug
      m droid
      stat $OUT/system/apex/com.android.runtime.debug/bin/dalvikvm
Bug: 124924906

Change-Id: I04e696602b776b383bf7198eb19220353df74192