[release-0.19] Commits to cherry pick

[bestander]

• [HMR] Improve Hot Loading gating [HMR] Improve Hot Loading gating #5339
• Add an Experiments class to enable/disable experiments. Add an 'Experiments' class to enable/disable experiments. #5446 (need to commment out HMR menu code)
• Fix for Android split builds: 342a763
• Cross-platform AppState: c2d75d7, [Android] AppState #5152
• Fix StyleSheet 'textAlign' for AndroidTextInput, f453e14 (Fix StyleSheet 'textAlign' for AndroidTextInput. Closes #2702 #4481)
• Fix Image crash on invalid source Uri 798acac
• Android Picker, ideally with the same API as iOS, 1dd5ba7.
• Android date and time pickers, 9a0539d, Update: can't cherry-pick because would need to cherry-pick a few commits before that related to Promises which look risky.
• Figure out what to do with react-native-xcode.sh, there's still the outstanding issue with 7ecb693 which we had to revert on 0.19.0-rc:. Update: this should help and probably worth cherry-picking: d3e4414( Improve react-native-xcode.sh integration #5518), maybe even Skip bundling for Simulator #5519 (not necessary)
• Update website/server/extractDocs.js with the new modules and APIs so they appear on the website

New Android modules in this release: CameraRoll, AppState, Date and time pickers, Picker

[mkonicek]

0.19 is out: https://github.com/facebook/react-native/releases/v0.19.0

[bestander]

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