Call SidecarCallback on main thread.

Call the SidecarCallback on the main thread to avoid deadlocks.
There is a deadlock where the SidecarCallback was running on a binder
thread and a deduplicator in WM jetpack was running on the main thread.
They would have some locks interwovend causing a deadlock. Running on
the main thread will eliminate this.

Flag: androidx.window.sidecar.flags.merge_extensions_sidecar
Bug: 347964255
Test: n/a - analyze the stack traces in the reported bug.
Change-Id: I0e19cd2a2f8b7a63388216fb3b299b0e48f0ae99