Move setAttachedClientForTesting() to DefaultImeVisibilityApplierTest

This is a mechanical refactoring CL, which moves

  InputMethodManagerService#setAttachedClientForTesting()

to DefaultImeVisibilityApplierTest, where it is being used.

This is test only change. There must be no observable behavior change.

Bug: 350386877
Bug: 352615651
Flag: TEST_ONLY
Test: atest FrameworksInputMethodSystemServerTests
Change-Id: I6c1c9f10d35f6bc1ed1f40640b8ff06434b3fe92