Migrate FlutterViewController to ARC (#55669)

Migrates `FlutterViewController` from manual reference counting to ARC. Eliminates use of scoped_nsobject and scoped_nsprotocol, and migrates ivars to property syntax where possible.

No semantic changes, therefore no changes to tests.

Issue: https://github.com/flutter/flutter/issues/137801

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
https://dart.googlesource.com/external/github.com/flutter/engine/+/3162aaad0086f874b68c538c98cc8d20b4e4233a

DEPS

diff --git a/DEPS b/DEPS
index 9089859..642b1f4 100644
--- a/DEPS
+++ b/DEPS
@@ -267,7 +267,7 @@
 deps = {
   'engine/src': 'https://github.com/flutter/buildroot.git' + '@' + 'c8f93f25a19cefaaeb64d4323e2fc8c9ccd20479',
 
-  'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + 'b7c3af43f90ac9aaf66dbd10b922c25addd9c3db',
+  'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + '3162aaad0086f874b68c538c98cc8d20b4e4233a',
 
   'engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '2202f10aa4642f7b956a093463633d5dc17254bf',
 

commits.json

diff --git a/commits.json b/commits.json
index 922209c..e4d997f 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "engine/src/flutter":"b7c3af43f90ac9aaf66dbd10b922c25addd9c3db",
+  "engine/src/flutter":"3162aaad0086f874b68c538c98cc8d20b4e4233a",
   "engine/src/flutter/third_party/dart":"2202f10aa4642f7b956a093463633d5dc17254bf",
   "flutter":"5a11904383d18e40c3c683061db7706a9f6feb6c"
 }
\ No newline at end of file