Fix setting state description of toggle button and switch

We should set the state description whenever textOn and textOff are
updated. Setting it multiple times doesn't hurt because the
setStateDescription method won't do anything if the state description is
the same.

Test: tested with talkback test app. Without the change, a toggle button
with textOn and textOff set won't have them announced as the state
before a click happens. After the change, a toggle button with textOn
and textOff set has them announced as state.

Change-Id: Ia1cc6218c8cd0125c123c9b650bb4963825db23a