Remove a redundant abstraction ImeVisibilityApplier

Perhaps the original intention was to use this interface to switch
multiple strategies, but in reality it did not happen.  People often
choose to experiment new behaviors by directly adding them to
DefaultImeVisibilityApplier (often behind the flag) rather than
creating a new implementation of ImeVisibilityApplier. Perhaps it
reminds me of PhoneWindowManager. Probably they are both too early
abstraction.

Let's remove it until we really start using it.

This is a purely mechanical refactoring CL. There must be no behavior
change.

Bug: 350101939
Test: presubmit
Test: atest DefaultImeVisibilityApplierTest
Flag: EXEMPT refactor
Change-Id: I72e54cb91fc09660e5fcd0dfcd9089c4419b62a7