Add tvOS to podspec supported platforms

[adamkoch]

Summary:

After upgrading my tvOS project to RN 0.60+, I found the new pod auto-linking feature wasn't working for async-storage on tvOS due to support not being specified for tvOS. Adding this to the podspec allows pod install to work correctly for tvOS.

Test Plan:

Try and use async-storage in a tvOS project on RN 0.60+. Note that pod install will throw an error (tvOS not supported). Update the podspec as per this PR and try again and it will work.

[krizzu]

Right, this looks like something we need for people using tvOS with autolinking. One question: Why you use version 9.2 instead of 9.0, like iOS does?

Cc @tido64

[adamkoch]

This is the version I tested/verified personally and that my environment was set up with. I believe when creating a new RN project the tvOS component that gets created has a target of tvOS 9.2, I'm not sure the reason why.

See:
https://github.com/facebook/react-native/blob/master/template/ios/HelloWorld.xcodeproj/project.pbxproj#L549

And original commit:
facebook/react-native@d625a72

[krizzu]

Right, thanks @adamkoch, looks good to me.