Add force suspend API to PowerManager.

Add a hidden system API (protected by DEVICE_POWER) that forces suspend,
ignoring any existing wakelock.  Add a shell cmd to trigger the API
to run.

Bug: 111991113
Test: 'adb shell svc power forcesuspend'
Change-Id: I5a258e1b7c8b1391fe1baf3930dd9d9af47235c9