cuttlefish-4.14: Enable CONFIG_DM_SNAPSHOT

The snapshot dm target is required to implement the Virtual-AB
mechanism.
Introduce CONFIG_DM_SNAPSHOT in arm64 and x86 cuttlefish defconfigs to
enable this feature.

Bug: 142527064
Test: kernel build
Signed-off-by: Alessio Balsini <balsini@google.com>

Change-Id: I69bc614509eaff259a2aa9195e7e1d406b36bbb2
Signed-off-by: Alessio Balsini <balsini@google.com>