The focal length slider now works over a remote connection. Also, the
slider for focus distance now controls the value in `ViewerSettings`
(which gets applied to Camera) rather than the value in
`DepthOfFieldOptions` (which is deprecated).
This adds libs/viewer to filament-utils-android and provides JNI
bindings for AutomationEngine.
This commit also changes the AutomationEngine to use PPM for screenshots
rather than PNG. The reasoning is that it is wasteful to include stb and
our two image-related libraries just to dump screenshots, especially for
a feature we might not use on Android.
