Share split screen utils with non-WM tests.

Extracted split screen activity methods from ActivityManagerTestBase
to be reused by tests not needing full ActivityManagerTestBase suite.
This promotes code sharing, simplifies maintenance, and streamlines test
activity launch logic.

Bug: 355452977
Test: mmm cts/tests/framework/base
Test: atest CtsUsageStatsTestCases:UsageReportingTest
Flag: TEST_ONLY
Change-Id: Icaba55771c24ca5cf08ca81895055c20650766dc