KeyguardPreviewRenderer: Drop usages of runBlocking

- This CL refactors KeyguardPreviewRenderer to eliminate runBlocking
  usage for `SurfaceControlViewHost` creation, using Handler.post
  instead
- Previously, the SurfaceControlViewHost was created within a
  runBlocking block on the main dispatcher. This blocks the
  calling thread, potentially causing UI jank or ANRs during Keyguard
  preview initialization.
- SurfaceControlViewHost creation and its disposal logic are now posted
  to the mainHandler. This allows the operation to be scheduled on the
  main thread's message queue, executing asynchronously without blocking
  the initiating thread. The class member "host" is now lateinit to
  accommodate its deferred initialization.

Bug: 423462317
Flag: com.android.systemui.do_not_use_run_blocking
Change-Id: Ic93c30d0af2ed5067711de2e7bca984d05ce3d5d