Mark version_defaults as readonly

These shouldn't be changed by anyone after this point, and this will
prevent random Android.mk from re-including this file.

Test: m nothing
Test: failed on hikey w/o https://android-review.googlesource.com/662764
Change-Id: I7ba4db3594af5b18b98fafee81b64531a40df396
Merged-In: I7ba4db3594af5b18b98fafee81b64531a40df396