Optimize Android implementations

[chrisbanes]

There's a lot of optimizations in this PR, which all add up. Most of it is just boring stuff:

• Stop creating Path instances every time. They're fairly expensive so creating new ones each time is wasteful. Fixed by adding a Pool allowing acquiring/release of instances. We now basically only ever keep areas.size + 1 paths in memory. Previously we'd create them each time an update which happened (so basically every layout 😬)
• Stop creating RenderNode instances every time. Hopefully you get the idea from above, same story. We now mutate our internal Effect instances (which hold the RenderNodes), rather than creating a new one every layout.
• Only invalidate drawing when required. Related to ☝️. As we now keep the same 'effect' instance, we can now keep track of what has changed in between 'updates', and therefore only invalidate when needed.
• Defer work to draw. Our update call can be called a lot (triggered by state updates), so we don't want to do update work every time it is called. Instead, we now set dirty flags and wait for the next draw call to lazily defer the work.

As I said, none of this is ground breaking stuff. However, this the kind of stuff which Modifier.Node allows us to do since we're no longer in declarative land.