Fix Power menu background vanish

 - Separated button & origin button use same HardwareBgDrawable caused
power menu background vanish when keyboard showing.
 - Let separated button use mSeparatedViewBackground & origin button use
 mListBackground to fix this bug.
 - Rename View mChild to mList.

Test: Manually, long press power key to launch power menu when keyboard
showing, power menu background should display normally.
Bug: 111101759

Change-Id: Ic08cabc63daebc4001c8eda751857d11cb0e6352