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: 112168722
Bug: 111101759

Change-Id: I7122ff52fd14021ac01b1ab9d64e7dde71fa7ab1
Merged-In: Ic08cabc63daebc4001c8eda751857d11cb0e6352
Merged-In: I7122ff52fd14021ac01b1ab9d64e7dde71fa7ab1