Add future_updatable to the apex module

The new property is used to mimic some of the build behaviors that
currently are applied only when an APEX is `updatable: true`. This
property is helpful to minimize the sudden impact (e.g. change in size)
when an APEX becomes updatable.

Currently, the behavior that this property mimics is the disabling of
the size optimization.

Bug: 207336449
Test: m
Change-Id: I500306afa1221625fda3e89a8d12d5d341791a1f