Move TestShellExecutor in multivalent to common

TestShellExecutor in unit tests and multivalent tests may conflict.
Move the multivalent version to a different package.

Bug: 390183899
Test: atest WMShellRobolectricTests
Test: atest WMShellMultivalentTestsOnDevice
Test: DesktopTasksLimiterTest builds in studio
Flag: EXEMPT updating unit tests
Change-Id: I98f4de3b44a0db7c9b60004fc9c0958448fdc821