Remove ImeTargetVisibilityPolicy

With the changes in [1] and [2] and the removal of the DefaultImeVisibilityApplier, the ImeTargetVisibilityPolicy is not needed as a separate class. We can move the reamining methods directly into WindowManagerInternal and DisplayContent.

[1]: I8e3a74ee579f085cb582040fdba725e7a63d6b85
[2]: I1399ec6c9e3f5ed70f02ba2326edd0c73eb930b4

Fix: 433659053
Test: atest InputMethodManagerServiceScreenshotTest
Flag: EXEMPT removing file
Change-Id: I004098c8e4a029fcc709e03bc375785e59c21d48