ANDROID: GKI: enable KFENCE by setting the sample interval to 500ms

This change effectively enables KFENCE in GKI kernels with a sample
interval of 500ms.
It is still possible to disable KFENCE at boot time using
kfence.sample_interval=0.

Bug: 176228452
Test: kernel builds, device boots normally
Test: TreeHugger

Signed-off-by: Alexander Potapenko <glider@google.com>
Change-Id: I69cb0fc8fdebba36c9166b275ad245240314d811