[memory observer] allow users to specify which screens they want to reduce memory from

Right now, clicking "Reduce memory" from the memory pressure prompt clears data from all other screens. Users may want to pick and choose the screens they would like to reduce memory from.

Follow up feature for https://github.com/flutter/devtools/pull/8998.

Start by reviewing the memory observer and the memory pressure prompt flow, along with the follow-up context in pull request 8998. Trace what the “Reduce memory” action currently clears. Done means users can choose which screens to reduce memory from instead of always affecting all other screens.

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