Remove support for legacy React Native versions

[AndrewJack]

Fixes: #841

v2 of code push only supports RN 0.43 and later. This means we no longer need to use reflection for backward compatibility.

[msftclas]

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[msftclas]

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[AndrewJack]

Any issue with clearing this before we attempt to load the bundle? We don't need the callback after this point.

We must call this before loadBundleLegacy() or it will go into an infinite restart loop.

[AndrewJack]

@sergey-akhalkov any chance of this being reviewed/ merged soon?

[max-mironov]

@AndrewJack - thanks for great contribution. I believe we should test it very careful before merging this changes.
Also I'vent got a chance yet to dive deeper into it but we have one PR that probably has some overlap with your changes, do you mind to have a quick look at it also to see if it somehow deals with your problem and if you think it ever make sense (or your changes will fix this item also). We really appreciate your experience here and your insight would be very valuable for us.

[AndrewJack]

@max-mironov I've had a look through your changes. I'll rebase my PR once yours has been merged.

[AndrewJack]

@max-mironov I've rebased after your changes were merged.

[max-mironov]

Hello @AndrewJack. Your changes looks good and reasonable to me! I've double checked all main cases (with different installModes) on emulator and on real device with signed apk (Android7) and haven't found any issues. We really appreciate your efforts and help here - thank you for this!

I'm good with merging this stuff but probably @sergey-akhalkov could also have a look at this before we incorporate this changes to Master.