Add feature flag for app hibernation.

Add feature flag for app hibernation so that we can begin implementing
behind a feature flag.

We use DeviceConfig as per instructions at go/android-flags-setup so
that we may eventually roll this out to dogfood devices using P/H and
Mendel.

This is the downstream merge of http://aosp/1542244

Bug: 175829330
Test: Builds
Change-Id: I27b4bdd9b6ed25b06fc685183f0dc872094f1b5c