Remove mock for static method and use resource overrides

For one test case we want to have desktop mode disabled.
Previously this was done via extended mockito and mocking a static
method.
Remove extended mockito and use TestableContext resource override
support instead.

Bug: 390705096
Test: atest com.android.wm.shell.desktopmode.DesktopModeVisualIndicatorTest
Flag: EXEMPT fixing test
Change-Id: I096043c94fe78c9b3d04c011a4f970a9429c4491