This change updates the platform runners in the benchmarks subdir.

I've attempted to replace the entire group with new code as if the project were recreated with flutter create so that they don't become a Frankenstein of minor changes to make it work again, but just plain old generated code that can be replaced and updated in place.

For https://github.com/flutter/flutter/issues/24712

This test verifies that https://github.com/flutter/engine/pull/6923 will speedup the average rasterize time of this test from ~150ms to ~10ms

Please see non-auto-generated files in https://github.com/flutter/flutter/pull/25381/commits/37b21d9fb4756a8d8d12ae0def1d0640740c8818

* Add iOS template

* Android

* Let the engine reset the theme without the activity knowing

* Small tweak

* Replace assets with different vectors

* Let the template hookup have no actual image assets

* Add back placeholder assets with 1px transparent pngs

* Fix drawable xml

* clean up an extraneous line in the storyboard xml