Separate mIsLeashInitialized from isLeashReadyForDispatching

For non-IME providers, if the leash is initialized, it is ready for
dispatching to the control target. But ImeInsetsSourceProvider has its
own logic to decide if a leash should be dispatched to the target.

This CL separates mIsLeashInitialized from isLeashReadyForDispatching.

Bug: 298172246
Bug: 298018626
Bug: 356783416
Flag: EXEMPT bugfix
Test: InsetsSourceProviderTest
Change-Id: I199f8e9434dcab7361ec69c43b7e66d7560e0f65