Play Zulip's distinctive notification sound on Android

[gnprice]

In zulip-mobile we have a distinctive notification sound we created a couple of years ago, which we use on Android by default. We also have a suite of variant sounds which we provide for the user to choose from if they use the system notification settings.

We should do the same here. My initial implementation of #320 / #122 won't do so, in order to manage complexity.

We should also eventually do so on iOS. That will be tracked separately, because it's a feature we don't yet have in zulip-mobile either: zulip/zulip-mobile#3101 .