Remove InputManager and DisplayManager from ViewRootImpl

When investigating a memory leak, I noticed how DisplayManager is
getting a special path in `DecorContext`, specifically because of
`ViewRootImpl` holding references to the managers. Turns out that
those references are not really needed if we rely on the `Global`
managers.

Bug: 278349152
Test: ABTD
Change-Id: I13d2ad2d7b439e9e1467cbde053c7f41c5416432