Fix WindowSessionSupplier Tests

The WindowSessionSupplier requires a Lock to get
the reference to WindowSession object which is stored
in a static variable in WindowManagerGlobal.

Extending ShellTestCase allow the test to handle
WindowSession correctly.

Flag: com.android.window.flags.app_compat_refactoring
Fix: 394215936
Test: atest WMShellUnitTests

Change-Id: Ib6e7d7a77d9682d96437af4a3618545e7c2bd6f7