Synchronizing loader preload to avoid launcher getting stuck in loading state,

which could be caused by the following sequence of events

  1) Starts preload => execution moved to background thread
     a) check if loader is running
     ... execution moved to ui thread

  2) Launcher starts
  3) Cancels any running loader and starts a new loader
     .... Execution on background thread
    b) Cancels any running loader and starts a new loader

Synchronizing (3), and [a, b] under same lock would avoid this case
Bug: 73399920

Change-Id: I6b01f797fd6f4a2e5b3c078bb374ad40fcc311c8