28937 Cherry-pick #28846 and #28743 into flutter-2.6-candidate.11 (platform-ios, platform-android, cla: yes, needs tests, embedder)

28938 fixed android unit tests (rotted before CI is running) (platform-android, cla: yes, waiting for tree to go green)

28977 Remove some deprecated APIs from the Android embedding (platform-android, cla: yes, waiting for tree to go green, needs tests)

28987 Defer setup of the default font manager if the embedding prefetched the font manager (platform-android, cla: yes, waiting for tree to go green, needs tests)

29013 Expose updateSystemUiOverlays in FlutterActivity and FlutterFragment (platform-android, cla: yes, waiting for tree to go green)