Export flow type for ScrollView Deceleration Rate

Summary:
## Changelog:
[General] [Changed] Export Flow type for deceleration rate for use in other files to keep deceleration rate prop values consistently typed

Reviewed By: lunaleaps

Differential Revision: D32989199

fbshipit-source-id: 2e2fef0721de0d0eb60aaefdbb635788bfc8c1f1

Author: Amy Huyen

Libraries/Components/ScrollView/ScrollView.js

@@ -161,6 +161,7 @@ export type ScrollViewImperativeMethods = $ReadOnly<{|
   >,
 |}>;
 
+export type DecelerationRateType = 'fast' | 'normal' | number;
 export type ScrollResponderType = ScrollViewImperativeMethods;
 
 type IOSProps = $ReadOnly<{|
@@ -473,7 +474,7 @@ export type Props = $ReadOnly<{|
    *   - `'normal'`: 0.998 on iOS, 0.985 on Android (the default)
    *   - `'fast'`: 0.99 on iOS, 0.9 on Android
    */
-  decelerationRate?: ?('fast' | 'normal' | number),
+  decelerationRate?: ?DecelerationRateType,
   /**
    * When true, the scroll view's children are arranged horizontally in a row
    * instead of vertically in a column. The default value is false.