Make ActivityStackSupervisor instrumentable for unit tests.

Currently, the ActivityStackSupervisor passes an instance of itself
during construction to a number of its member variables. Passing
a self reference at this time leads to an incomplete object being
passed. In the case of testing, it's possible for the instance to be
copied after construction for instrumentation, leading to the
original instance being retained.

This changelist addresses this by moving the creation of these
variables to a separate initialize methods. In addition, the affected
variables accessed outside the class are now behind accessors to
prevent their modification.

Bug: 64750076
Test: bit FrameworksServicesTests:com.android.server.am.ActivityStarterTests
Test: bit FrameworksServicesTests:com.android.server.am.ActivityStackSupervisorTests
Test: bit FrameworksServicesTests:com.android.server.am.ActivityStackTests
Test: bit FrameworksServicesTests:com.android.server.am.LaunchingBoundsControllerTests
Test: bit FrameworksServicesTests:com.android.server.am.LaunchingActivityPositionerTests
Test: bit FrameworksServicesTests:com.android.server.am.LaunchingTaskPositionerTests
Test: runtest --path frameworks/base/services/tests/servicestests/src/com/android/server/am/RecentTasksTest.java
Change-Id: I19647b24925b82db0ab0955585d7588a28e00f94