Added support for resizing background color for an app window

R.styleable.Window_windowResizingBackground allows an activity to
specify the background drawable that should be used when it is being
resized in multi-window mode. If unset, the system will try to use
R.styleable.Window_windowBackground if set, then
R.styleable.Window_windowBackgroundFallback if set. Otherwise, the
system default resizing background color set by
R.integer.config_windowResizingBackgroundColorARGB.

Also, use decor title color as caption background color when resizing
instead of black.

Bug: 24534744
Change-Id: I83313865b4044b976ebc78d598e14e17e0f37212