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

This reverts commit 88c9dddd.

Bug: 37815285
Bug: 62525809
Test: Tested with ag/2400523 and ag/2400524. Booted pixel phones,
      checked the location of prop.default, verified the symlink,
      checked a few properties via adb shell 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
      Booted to recovery and ran 'adb sideload' successfully.

Change-Id: I2ddf287f42360d1a7c7a22f08e7b9e203e7e603a