Stop preloading recent tasks in SysUI when bound to overview service

- When bound to the overview service, trim the entire cache in the SysUI
  recents cache. This is only useful until we remove the recents code
  in SysUI entired.

Bug: 74198066
Test: Manual, dump hprof and ensure no references to thumbnails
Change-Id: I694c917f2ed2b84c842c2815c4071947437446ed