allow use external typeface in native code

Summary:
Expose method to implement changing font family cache.  Like ide suggested in #4420 , this will helpful for using remote font file (use `Typeface#createFromFile` to load downloaded font file).
iOS's CoreText  already allow this in native code.
Closes #4696

Reviewed By: bestander

Differential Revision: D2911762

Pulled By: andreicoman11

fb-gh-sync-id: a931e2e711dd94fa0df6fdd066827756d862a4ba

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

@@ -72,6 +72,25 @@ public static ReactFontManager getInstance() {
     return typeface;
   }
 
+  /**
+   * Add additional font family, or replace the exist one in the font memory cache.
+   * @param style
+   * @see {@link Typeface#DEFAULT}
+   * @see {@link Typeface#BOLD}
+   * @see {@link Typeface#ITALIC}
+   * @see {@link Typeface#BOLD_ITALIC}
+   */
+  public void setTypeface(String fontFamilyName, int style, Typeface typeface) {
+    if (typeface != null) {
+      FontFamily fontFamily = mFontCache.get(fontFamilyName);
+      if (fontFamily == null) {
+        fontFamily = new FontFamily();
+        mFontCache.put(fontFamilyName, fontFamily);
+      }
+      fontFamily.setTypeface(style, typeface);
+    }
+  }
+
   private static
   @Nullable Typeface createTypeface(
       String fontFamilyName,