Custom any value generator

[hrach]

Closes #642

[hrach]

Hi @Raibaz this is probably ready to go.

• first commits properly reads the nullability
• the second introduces mutability for any value generator and opens the api a bit
• the third commit is a fix from my previous PR changing this, sadly it passed the tests but running in my unit-tests (not-android test) it was somehow missing - adding copies to proper places
• the fourth commit is fixing reflection exception, I was unable to find repro case as this was pretty much complicated code (generics, interface with inheritance, reactive chains)

I'd tried to fix the android runner as discussed here: ReactiveCircus/android-emulator-runner#160 but doesn't seem to work.

[Raibaz]

Whatever, merging this and in case the android builds still fail I'll address them on master.

Thanks a lot for all the work you put into this!

[hrach]

Thank you a lot! I have to say your maintainer's response time/attitude exceeded my expectations a lot!

[cbxp]

Cannot find any documentation on how to substitute own null-returning AnyValueGenerator...

[mafo5]

Cannot find any documentation on how to substitute own null-returning AnyValueGenerator...

The headline is misleading! The PR extends the relaxed=true behavior to the AnyValueGenerator as well.