Add ImeTargetVisibilityPolicy#isReadyComputeImeParent

Wrap some complex reparent ime parent logic from
DisplayContent to ImeTargetVisibilityPolicy.

We only move the logic into a static method, there
shouldn't have behavior changes.

Bug: 258048231
Test: presubmit
Change-Id: I344333a3d0eedf780cd47152e0f4c20022cb5fed