Improve fixed header in overlays on iOS

Fake fixed header by wrapping overlay content in a div and scrolling
within that div instead (on iOS only). Modify content div's height
when virtual keyboard is open/closed or window resized.

This also improves scrolling in notifications overlay on iOS.

Tested on iOS 7, recent Chrome for Android and Android Browser 2.3.

Bug: 67390

Change-Id: I05f11246ca90a1ce3f741fd93194a827528cd597