Use ro.build.version.extensions.r

The sdk_info prop is mostly there as a communication channel
if we need to channel more information in the future. For now,
using ro.build.version.extensions.r (which is "stable") is sufficient.

Bug: 137191822
Test: boot and getprop ro.build.version.extensions.r
Change-Id: I630c235fd98eaeda9086fa490914e56556b18706