Reduce IME navbar DeadZone by half

In order to reduce touches being consumed by Deadzone and letting them
reach navbar, this CL reduces its dimensions by half.

This is an attempt to unblock us from deadZone consuming touch events.
In the long run, DeadZone should be replaced with NearestTouchFrame.

Bug: 364856257
Test: atest CtsInputMethodTestCases
Flag: android.view.inputmethod.fix_navbar_deadzone
Change-Id: Ib4050a4928427c8746fa5a9737b889aca8c664e5