Switch widget: Fix ON/OFF option update error

When calling setChecked() from a listener which is invoked by
the Switch toggle, setChecked() will be called recursively,
causing the value of mThumbPosition overridden by the outer
setChecked(). This make the nested setChecked() unable to update
the "Thumb" position correctly.

Set mThumbPosition to sync up with the exact checked state of
the Switch to avoid update error.

Issue 34748

Change-Id: Id4aa389114dcc590430b11626c80d29c9c6016a0
Signed-off-by: Yu Haonong <yuhaonong@gmail.com>