Formalize DeviceIdle's exitIdle call.

In preparation for DeviceIdle mainlining, we have to define stable APIs
for other modules and the rest of the system to use. This introduces a
stable API for the exitIdle call.

Bug: 142281756
Bug: 142393582
Test: atest NotificationManagerTest
Test: atest PhoneWindowManagerTest
Change-Id: I79ea2b34d9abb2bb44575e98cc923d6383419230