Remove hideCurrentInputMethod from RecentsAnimationController

Since CL[1] migrate hide IME logic when quick switching split-screen
task to InputMonitor.

As a result, remove unused API since it would be no longer to
expose hideCurrentInputMethod for launcher to handle gesture.

[1]: Ibfcd48e623336c4690b71c4db0ce1ad8f5b26fc9

Bug: 166736352
Bug: 193990612

Test: manual test as steps
1) launch any apps with focusing an editor
2) from overview, select any app to enter split-screen mode
3) taping the editor to show the keyboard
4) swipe up to overview or quick switch app tasks
5) expect the keyboard will be hidden when starting the gesture.

Change-Id: I76b93af015db098e836795f72f31b663238d9a47