Change some initializations in am tests for ARC

This CL upstreams the following changes in ARC. These
changes do not affect the behavior on phones, but should be
upstreamed to avoid future merge conflicts.

Changes to be upstreamed:
- Fix com.android.server.am.TaskRecordTests(ag/4488735)
This Cl initializes task.realActivity, which is used in
ActivityManagerInjectorArc#onTaskCreated().

- Create user as guest in TaskPersisterTest(ag/4477502)
In ARC, the maximum number of supported users is one, which is
different from the ones of most phones (more than 4). This prevents
TaskPersisterTest from creating another user for test. However,
since guest users can be added as much as possible, we create guest
user in the test.

- Change taskId in ActivityTestBase from 0 to 1(ag/4474682)
In ActivityTestBase, taskId is set as 0 by default. However, since
0 is reserved in ARC, currently assertion failure occurs in
wayland_service.cpp. This CL shifts the id to 1 and makes all the
tests in ActivityStackTests pass.

Bug: 72843139
Test: atest TaskPersisterTest
Test: atest TaskRecordTests
Test: atest ActivityStackTests
Change-Id: I6ba49547ae3b34a752e401150caeb1d786f199cf