Shortcut Helper: dismiss when switching to a different user

Moves the logic to PhoneWindowManager, to avoid having it in 3 different
shortcut helper implementations: old phone version, old tablet version,
and new version.
PhoneWindowManager already dismisses the keyboard shortcut helper in
other scenarios, therefore it is a good fit.

Fixes: 343884295
Flag: EXEMPT simple change
Test: atest WmTests:PhoneWindowManagerTests
Test: Manually with old and new implementations
Change-Id: Ia8ef8a2c1337a0bb89ecfb9f7c5319e326646d95