Refactor WindowTestsBase to use one-task depth activities

Refactor following part to use one-task depth activities instead of
2-level tasks.

- WindowTestsBase#createTestActivityRecord
- ActivityBuilder

Bug: 155824575
Bug: 144248496
Test: atest WmTests
Change-Id: Ia3e5796af70b7e59744c7d744b46280a211040a4
36 files changed