Use true present fence times in PowerHintSession duration calc am: c6b9d38c27

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/native/+/18976901

Change-Id: I2be5a6c4f27980c32e30ce50af457fa730f302ff
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>