Simplify task queues locking mechanism (#16477)
We now have one mutex guarding all accesses to the underlying task heaps. This simplifies the more granular but bug prone mechanism of having striped locks. This also re-enables GPUThreadMerger tests that are currently disabled due to their flaky nature. The scenario that gets fixed by this change is as follows: 1. Thread-1: We lock `queue_meta_mutex_` and grab locks on `queue_1` and release the meta mutex. 2. Thread-1: We add an Observer on `queues` object. 3. Thread-2: We lock `queue_meta_mutex_` and grab locks on `queue_2`. 4. Thread-2: We try to dispose all the pending tasks on `queue_2` which calls `erase` on `queues`. The above situation is not thread safe without having 1 lock. Note: This increases the contention on one lock and could potentially be bad for perf. We are explicitly making this trade-off towards reducing the complexity. Fixes: https://github.com/flutter/flutter/issues/49007
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