Merge "Move variable assignment handling to generation context" am: 7cd255fa

Original change: https://android-review.googlesource.com/c/platform/build/soong/+/2057162



Change-Id: I0dcd663160c05817d322010203eb9bf0efefae5a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>