Reverts "Manual engine roll to 6884e83 (#150733)" (#150746)

Reverts: flutter/flutter#150733
Initiated by: gmackall
Reason for reverting: This breaks apps that use plugins that use `compileSdk` 31, some of which we use in our postsubmit (so it blocks the tree).
Original PR Author: gmackall

Reviewed By: {jason-simmons}

This change reverts the following previous change:
Manual engine roll to https://github.com/flutter/engine/pull/53532, because the roll requires lockfile generation.
https://dart.googlesource.com/external/github.com/flutter/flutter/+/778eaf6c1dcaba7a433a66cbce25385d9db7877d

DEPS

diff --git a/DEPS b/DEPS
index ac642ce..84e36a7 100644
--- a/DEPS
+++ b/DEPS
@@ -284,7 +284,7 @@
 
   'engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '75c15c1e2e45397ed1e1984232165ff68ebfa622',
 
-  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'ca116ee535502e7718ff83b0d15282697126ed5b',
+  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '778eaf6c1dcaba7a433a66cbce25385d9db7877d',
 
   'engine/src/flutter/third_party/depot_tools':
   Var('chromium_git') + '/chromium/tools/depot_tools.git' + '@' + '580b4ff3f5cd0dcaa2eacda28cefe0f45320e8f7',

commits.json

diff --git a/commits.json b/commits.json
index 637b462..573c602 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "engine/src/flutter":"6991653cbe279e881de4047ed81ee7400bef5e00",
   "engine/src/flutter/third_party/dart":"75c15c1e2e45397ed1e1984232165ff68ebfa622",
-  "flutter":"ca116ee535502e7718ff83b0d15282697126ed5b"
+  "flutter":"778eaf6c1dcaba7a433a66cbce25385d9db7877d"
 }
\ No newline at end of file