Add a private IME option to suppress the gesture floating preview text.

If the option defined by Constants.NO_FLOATING_GESTURE_PREVIEW is set
for a text field via EditorInfo.privateImeOptions, the floating gesture
preview will be suppressed regardless of the settings preference.

Feature request:
bug: 17325039

Change-Id: I1a1df194369e8f2d76abd9d84939a057f55f4a1c