Add `offsetTranslationX` for the AddDesktopButton

Apply the `offsetTranslationX` to the AddDesktopButton inside
`updatePageOffsets`, so the button can be set to offscreen
when needed.
This requires us to differentiate the `gridTranslationX` and
the `offsetTranslationX` for this button as well.

Flag: com.android.window.flags.enable_multiple_desktops_frontend
      com.android.window.flags.enable_multiple_desktops_backend
Fix: 389209282
Test: Manual
Change-Id: Id157c41577b2f47b220558b353f07a4ecb0ec3b5