Move /default.prop to /system/etc/prop.default

This patch tries to fix the problem where the default properties need
to go with the system image especially on non-AB devices where
/default.prop is on the ramdisk image. A symlink is created at
/default.prop for backward compatibility.

Bug: 37815285
Test: Tested with ag/2376383. Booted pixel phones, checked the location
      of prop.default, verified the symlink, checked a few properties
      (via adb shell getprop) and manually tested a few apps (Camera,
      Maps etc).
      sign_target_files_apks.py was tested with:
      sign_target_files_apks -o -e DynamiteLoader.apk= -e DynamiteModulesA.apk= \
          -e DynamiteModulesB.apk= -e DynamiteModulesC.apk= -e DynamiteModulesD.apk= \
          -e GoogleCertificates.apk= out/dist/*-target_files-*.zip signed-target_files.zip

Change-Id: I1a9a2ba49c8252afc13ced3dea71253afbd3091e