Reduce Ring Buffer Default Sizes (and lower for Android)

[benjchristensen]

• This adds platform dependent checks to lower the buffer sizes on Android to use less memory. It also reduces the normal defaults as perf testing showed we didn't need to be as high.
• This adds System properties for overriding the defaults, rx.ring-buffer.size and rx.indexed-ring-buffer.size

Fixes #1820

[benjchristensen]

Going to merge to allow another release candidate and usage. Please provide feedback on whether this works on Android as expected or has problems in any way.

Also let me know if the property names are okay for overriding.

/cc @yogurtearl @mttkay @headinthebox @JakeWharton @neerajrj

[mttkay]

This sounds sensible to me. We never ran into problems with this, as our sequences are typically quite small (often a single item or a batch of items delivered in a single result), since in most cases for us there is no desire in processing e.g database records or items from a web API fetch one at a time, but rather together as a group, mostly due to the way Android processes view updates (it would be very inefficient to schedule a redraw for, say, every list item emitted.)

[JakeWharton]

Yep. 👍

[yogurtearl]

Works for our needs. 👍