Introducing activityType window configuration.

Used to represent the current activity type of the window
configuration which can be undefined, standard, home, recents, or
assistant. This information if currently represented in a few ways in
the window heirarchy e.g. with stack id or with fields in tasks and
activities. This change allows us to represent the idea in a more
structure way without overloading concepts like stack ids.

Test: bit FrameworksServicesTests:com.android.server.wm.ConfigurationContainerTests
Test: bit FrameworksServicesTests:com.android.server.wm.WindowConfigurationTests
Test: go/wm-smoke
Change-Id: I7d7283c72e806972eeb3a8dab448f195dd6cd811