Add workaround for androidx toolchain issues: try to load main dispat…

[qwwdfsad]

…cher via Class.forName(..) if ServiceLoader failed to find dispatcher

Fixes #657

[JakeWharton]

Why this over a @Keep (with actual/expect) or embedding the ProGuard rules inside the jar?

[qwwdfsad]

@JakeWharton

Why this over a @Keep (with actual/expect)

I don't want to have @Keep in core module API (especially if we will make MainDispatcherFactory public at some point), even in compileOnly mode.

or embedding the ProGuard rules inside the jar

To avoid toolchain issues we are not aware of. E.g. there is a lot of proguard rules for RxJava (not RxJava2) in the wild and these rules are not packed into RxJava itself. Class.forName solution "just works", but I have no such confidence about packed proguard rules.
If you are sure both old and new Android toolchains work properly with packed proguard rules and foresee no issues with that, I'd happily add it to the resources (along with -keepclassmembernames class kotlinx.** { volatile <fields>; }.

[JakeWharton]

You can avoid an annotation in the public API with an internal expect annotation in common and an internal actual typealias to the real one in JDK.

Only R8 works with embedded rules, but that shouldn't preclude their use. What's worse is shipping rules solely as documentation that require copy/pasting. Embed the rules in the JAR and have it be the source of truth. Have your documentation link to the rules for people using old toolchains. We should be optimizing for the future so that more and more people will get behavior that Just Works™.

[elizarov]

Style note: let's put an empty line after titles (it reads better in source, since it is actually a separate paragraph).