Summary:
**motivation**
Previously, size can only accept either 'small' or 'large'. And to obtain a custom size, scale transformation is used. This is to let users to possibly pass number value directly to define ActivityIndicator's size.
**Test plan**
I have also modified the current example to reflect the new size prop in action.
Closes https://github.com/facebook/react-native/pull/8935
Differential Revision: D3637910
fbshipit-source-id: 6b8e1d4504964916df327b2d3eaaef1bb8cd5112
63d15af18d