Android: implement Window::safe_area

[nicoburns]

Implements Window::safe_area for Android. This replaces the platform-specific WindowExtAndroid::content_rect method.

Android provides the top and right coordinates as offsets from the top left (0, 0) point rather than as insets from the nearest side. So these are subtracted from the outer size to normalize them according the existing semantics of Window::safe_area.

• Tested on all platforms changed
• Added an entry to the changelog module if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality