allow overriding spannedFromShadowNode in ReactTextInputShadowNode (facebook#24995)

Summary:
Motivation is the same as in facebook#24927 - when building a custom textinput (eg with rich text editing), one needs custom text processing logic. `ReactTextInputShadowNode` contains https://github.com/facebook/react-native/blob/6671165f69e37a49af8b709b4807f9049f7606c3/ReactAndroid/src/main/java/com/facebook/react/views/textinput/ReactTextInputShadowNode.java#L211

where an instance of `ReactTextUpdate` is created. For the custom use case, we'd like to just change the usage of [`spannedFromShadowNode()`](https://github.com/facebook/react-native/blob/6671165f69e37a49af8b709b4807f9049f7606c3/ReactAndroid/src/main/java/com/facebook/react/views/textinput/ReactTextInputShadowNode.java#L217) to our own implementation.

from there:
It's easy to subclass `ReactTextInputShadowNode` and override `public void onCollectExtraUpdates()` but the problem is that the method accesses private members. It also means overriding more code than necessary as we only care for `spannedFromShadowNode()`.

Solution might be changing the members to protected, but that seemed weird because there are already setters for them. Creating getters also seemed weird, as we'd end up having unused getters hanging around.

So the second way which I find nicer is changing `protected static Spannable spannedFromShadowNode(ReactBaseTextShadowNode textShadowNode, String text)` to just `protected` since that will allow subclasses to override just this behavior.

## Changelog

[Android] [Added] - allow custom spannedFromShadowNode in ReactTextInputShadowNode subclasses
Pull Request resolved: facebook#24995

Differential Revision: D15468066

Pulled By: cpojer

fbshipit-source-id: 73d5f0b9e06f3e02a03bf9db5effac62cecc80c4

Author: vonovak

ReactAndroid/src/main/java/com/facebook/react/views/text/ReactBaseTextShadowNode.java

@@ -226,7 +226,7 @@ private static void buildSpannedFromShadowNode(
   }
 
   // `nativeViewHierarchyOptimizer` can be `null` as long as `supportsInlineViews` is `false`.
-  protected static Spannable spannedFromShadowNode(
+  protected Spannable spannedFromShadowNode(
       ReactBaseTextShadowNode textShadowNode,
       String text,
       boolean supportsInlineViews,

ReactAndroid/src/main/java/com/facebook/react/views/textinput/ReactTextInputShadowNode.java

@@ -18,14 +18,9 @@
 import com.facebook.react.bridge.JSApplicationIllegalArgumentException;
 import com.facebook.react.bridge.ReadableMap;
 import com.facebook.react.common.annotations.VisibleForTesting;
-import com.facebook.react.uimanager.LayoutShadowNode;
-import com.facebook.react.uimanager.NativeViewHierarchyOptimizer;
-import com.facebook.react.uimanager.PixelUtil;
-import com.facebook.react.uimanager.ReactShadowNodeImpl;
 import com.facebook.react.uimanager.Spacing;
 import com.facebook.react.uimanager.ThemedReactContext;
 import com.facebook.react.uimanager.UIViewOperationQueue;
-import com.facebook.react.uimanager.ViewDefaults;
 import com.facebook.react.uimanager.annotations.ReactProp;
 import com.facebook.react.views.text.ReactBaseTextShadowNode;
 import com.facebook.react.views.text.ReactTextUpdate;