Migrate FlutterEmbedderKeyResponder to ARC (#52048)

Smart pointers support ARC as of https://github.com/flutter/engine/pull/47612, and the unit tests were migrated in https://github.com/flutter/engine/pull/48162.

Migrate `FlutterEmbedderKeyResponder` from MRC to ARC.  Clean up some header imports which made this seem like it depended on not-yet-migrated MRC files.

Part of https://github.com/flutter/flutter/issues/137801.
https://dart.googlesource.com/external/github.com/flutter/engine/+/6bd43ce038f5d578dac50b992e953b3e45bcca26

DEPS

diff --git a/DEPS b/DEPS
index b23e894..3eab020 100644
--- a/DEPS
+++ b/DEPS
@@ -291,7 +291,7 @@
 deps = {
   'engine/src': 'https://github.com/flutter/buildroot.git' + '@' + 'dcd71b5b237e1e58f2ad8a7e51bead0c2a3755a9',
 
-  'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + '0e56e3dffe43c022716caa06ac21a5ebba81fd41',
+  'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + '6bd43ce038f5d578dac50b992e953b3e45bcca26',
 
   'engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'f6a54972b3b981ca2dc482018f17297c96788c84',
 

commits.json

diff --git a/commits.json b/commits.json
index 3a6337c..5839a34 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "engine/src/flutter":"0e56e3dffe43c022716caa06ac21a5ebba81fd41",
+  "engine/src/flutter":"6bd43ce038f5d578dac50b992e953b3e45bcca26",
   "engine/src/flutter/third_party/dart":"f6a54972b3b981ca2dc482018f17297c96788c84",
   "flutter":"65e8ec214023f74153d38058e7dfb5c8770becf3"
 }
\ No newline at end of file