Add PLATFORM_VERSION_LAST_STABLE

This is a partial cherry-pick of Iaeefb35137de01c6ea57115d8830dd9bc30eccba
to prevent merge conflicts that are expected with the upcoming major
refactoring around sysprops.

The addition of the variable here doesn't cause any side effect because
it is simply ignored by the buildinfo.sh tool.

Bug: 117892318
Test: m
Change-Id: Ia805cb47ae07c40e91ede16bb6e5c4291de619b4
Merged-In: Iaeefb35137de01c6ea57115d8830dd9bc30eccba