Remove useZeroJankProxy flag

Cleanup code to remove the use_zero_jank_proxy flag.

Bug: 418839728
Bug: 418839448
Flag: android.view.inputmethod.use_zero_jank_proxy
Test: atest CtsInputMethodTestCases

Change-Id: Idbbb3c64511cb38275a4ad9c2bd7e33299ef4b56