Add app hibernation service stubs

Add app hibernation system service and manager class and start it when
app hibernation flag is enabled.

The actual API is currently stubbed and will be implemented in further
CLs.

Bug: 175829330
Test: adb shell device_config put app_hibernation
app_hibernation_enabled true
adb reboot
adb shell services list

Change-Id: Ide2758d3eaabae7d3df6356d4b6435c5137c554d