Add VariableFuncContext argument to VariableFuncs am: 1b457a5e10

Original change: https://android-review.googlesource.com/c/platform/build/blueprint/+/2286235

Change-Id: I2c04c17bea2f6632b36a58c773933b6bbfaf7d80
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>