Add PLATFORM_VERSION_FUTURE_CODENAMES.

PLATFORM_VERSION_ALL_CODENAMES isn't actually all the code names, just
the ones active in our current target. We need the list of all targets
so we can validate code names in NDK stub templates even if they're in
the future. Unfortunately, all_codenames has been baked into
build.prop for ages, so we're stuck with this name. Improve the
comments and add PLATFORM_VERSION_FUTURE_CODENAMES to cover to cover
the future targets.

Test: make checkbuild
Test: check $ANDROID_PRODUCT_OUT/system/build.prop
Bug: None
Change-Id: I81fed6f761e5362ef3050be9d1e3fb17739feadd