Don't log on each call when hibernation not enabled

Don't spam log when hibernation is not enabled. Instead, we log once
when we determine the hibernation enable state has changed.

Also add enablement check on new hibernation stats API.

Bug: 216621450
Test: manual
Change-Id: Id023f8ec97f243ad016ca7f8ad5515b05c0f8000
(cherry picked from commit 5e1fc990)
(cherry picked from commit b580eccca58104b977033f8e1ab369a114de8e3c)
Merged-In: Id023f8ec97f243ad016ca7f8ad5515b05c0f8000