[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.

首先查看内存观察器和内存压力提示流程，以及 pull request 8998 中的后续上下文。追踪“Reduce memory”操作当前会清除哪些内容。完成的标准是，用户可以选择从哪些屏幕减少内存，而不是始终影响所有其他屏幕。

由索引模型根据 Issue 内容生成。