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/2416542. 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
      Booted to recovery and ran 'adb sideload' successfully.
Change-Id: I1a9a2ba49c8252afc13ced3dea71253afbd3091e

tools/post_process_props.py

@@ -35,7 +35,7 @@ def mangle_build_prop(prop):
 def mangle_default_prop_override(prop):
   pass
 
-# Put the modifications that you need to make into the /default.prop into this
+# Put the modifications that you need to make into the /system/etc/prop.default into this
 # function. The prop object has get(name) and put(name,value) methods.
 def mangle_default_prop(prop):
   # If ro.debuggable is 1, then enable adb on USB by default
@@ -122,7 +122,8 @@ def main(argv):
   elif (filename.endswith("/vendor/default.prop") or
         filename.endswith("/odm/default.prop")):
     mangle_default_prop_override(properties)
-  elif filename.endswith("/default.prop"):
+  elif (filename.endswith("/default.prop") or # legacy
+        filename.endswith("/prop.default")):
     mangle_default_prop(properties)
   else:
     sys.stderr.write("bad command line: " + str(argv) + "\n")