3316 [camera] Suppress unchecked cast warning in java test (cla: yes)

3317 [image_picker] [integration_test] Fixes to make the tree green (cla: yes)

3318 Exclude null-safe plugins from testing on stable (cla: yes)

3319 Update analysis options for nnbd (cla: yes)

3320 Update Flutter SDK constraint to match templates; plugins A-C (cla: yes)

3321 Update Flutter SDK constraint to match templates; plugins D-G (cla: yes)

3322 Update Flutter SDK constraint to match templates; plugins I-P (cla: yes)

3323 Update Flutter SDK constraint to match templates; plugins Q-W (cla: yes)

3324 Merge null-safety plugins (cla: yes)