[macos] FlutterKeyboardManager memory leak fix

[sergiy-sc]

We are embedding Flutter into MacOS app, and noticed that there is a leak. leaks tool says there is a cycle reference:

12 (2.11K) ROOT CYCLE: <FlutterKeyboardManager 0x29ec55f40> [80]
       8 (432 bytes) __strong _primaryResponders --> ROOT CYCLE: <NSMutableArray 0x29ec560a0> [64]
          7 (368 bytes) ROOT CYCLE: <NSMutableArray (Storage) 0x29ec56140> [32]
             4 (224 bytes) ROOT CYCLE: <FlutterEmbedderKeyResponder 0x29ec56310> [80]
                1 (48 bytes) __strong _sendEvent --> ROOT CYCLE: <__NSMallocBlock__ 0x29ec56360> [48]
                   __strong [capture] --> CYCLE BACK TO <FlutterKeyboardManager 0x29ec55f40> [80]
                1 (48 bytes) __strong _pendingResponses --> <NSMutableDictionary 0x29ec563c0> [48]
                1 (48 bytes) __strong _pressingRecords --> <NSMutableDictionary 0x29ec56390> [48]
             2 (112 bytes) <FlutterChannelKeyResponder 0x29ec56450> [48]
                1 (64 bytes) __strong _channel --> <FlutterBasicMessageChannel 0x29ec564e0> [64]
       2 (1.55K) __strong _layoutMap --> <NSMutableDictionary 0x29ec56630> [48]
          1 (1.50K) <NSMutableDictionary (Storage) 0x123c34a00> [1536]
       1 (64 bytes) __strong _pendingEvents --> <NSMutableArray 0x29ec565f0> [64]

This patch uses weak pointer to self instead of implicit strong.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is [test-exempt]. See [testing the engine] for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the [CLA].
• All existing and new tests are passing.

[flutter-dashboard]

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If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

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[sergiy-sc]

Added unit test.

[cbracken]

This is fantastic; thanks for fixing. lgtm!