AutoBrightness: Remove handler's messages before send.

The contentprovider's callback 'onChange(...)' is asynchronous.
If one app set the 'screen_brightness' provider frequently over a period of time,
the contentprovider's 'onChange(...)' can't stop, just like 'Self-exciting oscillation'.
So, We need remove handler's messages before send.

Test: build pass, feature pass.
TestCase: set the 'screen_brightness' provider frequently, can use a
loop: for(i=0;i<1000;i++){put(...)}.

Signed-off-by: yangyankai1 <yangyankai1@xiaomi.com>
Change-Id: I346696c69623829d4dda372edaf35b6e3f147780