Remove restriction that only one callback with...

...PRIORITY_SYSTEM_NAVIGATION_OBSERVER can be registered.

This CL removes the API restriction that only one back callback with
PRIORITY_SYSTEM_NAVIGATION_OBSERVER can be registered at a time.
Starting with SDK 37, an unrestricted number of observer callbacks can
be registered. All registered observer callbacks will receive back
events when a system back navigation happens.

Bug: 424146227
Test: WindowOnBackInvokedDispatcherTest
Flag: com.android.window.flags.multiple_system_navigation_observer_callbacks
Change-Id: Ibed2eb513e7a3c80cab128f4f884d359a06c01f4