Use new storage for exported mode codegen

1, update aconfig cache update modifier to override storage backend to
ACONFIGD, so that when creating codegen context struct in java.rs,
use_device_config will be strictly false.

2, update the ensure! statement in feature impl template file selection
that it must not use device config for exported mode. As 1 should have
guaranteed this.

Bug: b/406508083
Test: atest -c
Change-Id: Iea770b5a32be49b22ca19f0f9f5930baf3f9699c