Fix smooth scrolling on old devices (SDK >=16) (#24545)

Summary:
React Native Environment Info:
    System:
      OS: Linux 4.15 Ubuntu 18.04.1 LTS (Bionic Beaver)
      CPU: (4) x64 Intel(R) Core(TM) i5-7300U CPU @ 2.60GHz
      Memory: 1.12 GB / 15.55 GB
      Shell: 4.4.19 - /bin/bash
    Binaries:
      Node: 8.10.0 - /usr/bin/node
      Yarn: 1.13.0 - /usr/local/bin/yarn
      npm: 3.5.2 - /usr/bin/npm
    SDKs:
      Android SDK:
        API Levels: 16, 19, 22, 23, 24, 25, 26, 27, 28
        Build Tools: 23.0.1, 23.0.3, 25.0.0, 25.0.2, 25.0.3, 26.0.1, 26.0.3, 27.0.3, 28.0.2, 28.0.3
        System Images: android-16 | Google APIs Intel x86 Atom, android-19 | Google APIs Intel x86 Atom, android-24 | Google Play Intel x86 Atom, android-27 | Google APIs Intel x86 Atom, android-28 | Intel x86 Atom_64, android-28 | Google APIs Intel x86 Atom, android-28 | Google Play Intel x86 Atom
    npmPackages:
      react: 16.8.6 => 16.8.6
      react-native: git+https://github.com/facebook/react-native.git#v0.59.5 => 0.59.5
    npmGlobalPackages:
      react-native-cli: 2.0.1
      react-native-git-upgrade: 0.2.7

The workaround implemented in #21117 tries to fix
https://issuetracker.google.com/issues/112385925 scroll direction (according to the latest comments, the scroll direction problem has been reverted in security patches so not sure if the workaround is still valid).

But... proposed solution in fling method is using signum which leads to zero computedVelocityY in case of zero mOnScrollDispatchHelper.getYFlingVelocity() on old devices(Samsung s4 mini) even when real velocityY is non zero

```
final int correctedVelocityY = (int)(Math.abs(velocityY) * Math.signum(mOnScrollDispatchHelper.getYFlingVelocity()));
```

Proposed solution is to take signum from original velocityY in case of zero
```
float signum = Math.signum(mOnScrollDispatchHelper.getYFlingVelocity());
if (signum == 0) {
  signum = Math.signum(velocityY);
}
final int correctedVelocityY = (int)(Math.abs(velocityY) * signum);
```

The symptoms are the same as described in issue #22925, but proposed workaround doesn't work.

[Android][fixed] - Fix smooth scrolling on old devices (SDK >=16)
Pull Request resolved: #24545

Differential Revision: D15044834

Pulled By: cpojer

fbshipit-source-id: 3f523eb1a438df774e22387aecded433b9031ab9

Author: Pavel Rotek

Diff for: ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactScrollView.java

@@ -314,8 +314,11 @@ public void fling(int velocityY) {
     //
     // Hence, we can use the absolute value from whatever the OS gives
     // us and use the sign of what mOnScrollDispatchHelper has tracked.
-    final int correctedVelocityY = (int)(Math.abs(velocityY) * Math.signum(mOnScrollDispatchHelper.getYFlingVelocity()));
-
+    float signum = Math.signum(mOnScrollDispatchHelper.getYFlingVelocity());
+    if (signum == 0) {
+      signum = Math.signum(velocityY);
+    }
+    final int correctedVelocityY = (int)(Math.abs(velocityY) * signum);
 
     if (mPagingEnabled) {
       flingAndSnap(correctedVelocityY);