Create platform API for first frame callback. Use for defer hiding splash screens on Android and iOS (#3956)

* Add back launch screen view until first frame on iOS

* improvements

* Move callback plumbing from ios surfaces to the gpu rasterizer. Didn’t wire java JNI yet.

* Android JNI

* Fix ios reference count and let android engine manage a view on top with launch screen

* Hook up Android activity and view

* review notes

* review notes

* Move thread switching upstream. Use weak references for callbacks.

* Some clean up

The bitmap used to hold a prerolled image in the raster cache must have an
integer width/height in pixels.  However, the picture used to draw the image
may have a non-integer width/height in physical coordinates and thus not
completely fill the bitmap.  The compositor should only select the subset
of the bitmap that reflects the portion filled by the rendered picture.

Without this, the cull rect calculated by an SkPicture may be truncated
to integer pixel coordinates.  The raster cache relies on a precise cull
rect in logical coordinates.

See https://bugs.chromium.org/p/skia/issues/detail?id=6954

* Revert "Revert "Build standalone dart sdk as part of Flutter engine. (#3954)" (#3970)"

This reverts commit 86e007d9.

* Add 7-zip download as a hook

* Advance buildroot dependency

* Fix buildroot dependency

This reverts commit a5bac1ee.

This doesn't work on Windows.

* Update dart deps and add create_sdk for host builds

* Set and use is_flutter flag

* Update to make it compatible with latest changes in https://codereview.chromium.org/2998503002/

* Fix deps

* Roll dart dependency to pick up new arguments

* Merge

* Update deps

* Update license

* roll Dart to 21eddc429e9c7260de00a9af7cad94baba830f59

* update licenses

Roll Dart to 69817eea182384b40ef658d578fd49cac00c5874

On linux the licenses tools progress reporting was cluttering the screen with extra characters and sometimes was missing the last progress update.

* Call weak_ptr.lock(), which returns a null shared_ptr and does not throw
* IsViewInvalid was inverted

Both dart:core and dart:_internal export Symbol. 

This is silently ignored by the VM but some tools (e.g. fasta) report a warning when processing dart:ui sources.

See https://github.com/dart-lang/sdk/issues/30127 for more details.

On emulators, render onscreen (with warnings) if any of the MakeS32 based SkSurface creation calls fail. (#3947)

* Mention Goma for Googlers in CONTRIBUTING.md

* review comments

This updates our compilers, GN and Ninja binaries.

Fixes https://github.com/flutter/flutter/issues/11352

The new `dart_observatory_pub_packages_rev` dependent on the rolled `dart_revision` required the update of the licensing files

Fixes https://github.com/flutter/flutter/issues/11437

Now, frame requests past the pipeline depth will never be deferred due
to back pressure. This backend is only used in the test runner.