chore: upgrade to rxdart 0.28.0

[arturograu]

Upgraded to rxdart since the apps that have installed the 0.28.0 cannot compile if are depending on this package.
Closes #134

[arturograu]

This was the only way to make the example compile since just_audio depends on audio_session. So audio_session cannot be upgraded to rxdart 0.28.0 until just_audio upgrades.

I suggest creating a pre-release with the content of this PR so we can upgrade just_audio at least depending on this pre-release. Then we can change this dependency back into the official just_audio and do the official release.

[ryanheise]

Hi @arturograu . Thanks for working on a PR, although it has a few issues that would prevent me from merging it:

• includes changes to iOS configuration files which are not pertinent to the rxdart fix.
• you have increased the lower bound for the rxdart constraint unnecessarily. We want to be able to support the whole range from the lowest possible version rxdart to the highest possible version of rxdart. You can see this approach from the existing code.

I will work on a fix.

[ryanheise]

v0.1.20 is now published bumping the upper bound of the rxdart constraint.