Reduce overdraw when using the flag SLIDING_CONTENT

When the method attachToActivity is called, currently if the content view has no background, it copies the background of the Window and results in overdraw. 

If the windows background is a color, the proposed changes will copy the color and only draw the background when the menu is sliding in / out. This results in no overdraw and speeds things up.

Currently the fix is only in place when the flag SLIDING_CONTENT is used.