textinput: Maintain cursor position when secureTextEntry toggles.

Summary:
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On current [master](https://github.com/facebook/react-native/tree/8235a49a33cc8e84cd4ba1cc15bc09eed6712b4c), text input cursor resets to start when `secureTextEntry` prop toggles on Android. This motivate me to maintain position when `secureTextEntry ` prop toggles for better user experience.

On current [master](https://github.com/facebook/react-native/tree/8235a49a33cc8e84cd4ba1cc15bc09eed6712b4c)
![ezgif com-video-to-gif-3](https://user-images.githubusercontent.com/18511177/35776882-bdc3b182-09ca-11e8-8f4e-218fae0a24a1.gif)

On this PR
![ezgif com-video-to-gif-4](https://user-images.githubusercontent.com/18511177/35776883-be082d94-09ca-11e8-9424-6164110bdf03.gif)

[ANDROID] [BUGFIX] [TextInput] - Fix: cursor positions resets to start on toggling `secureTextEntry` prop.

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  CATEGORY
[----------]        TYPE
[ CLI      ]   [-------------]      LOCATION
[ DOCS     ]   [ BREAKING    ]   [-------------]
[ GENERAL  ]   [ BUGFIX      ]   [-{Component}-]
[ INTERNAL ]   [ ENHANCEMENT ]   [ {File}      ]
[ IOS      ]   [ FEATURE     ]   [ {Directory} ]   |-----------|
[ ANDROID  ]   [ MINOR       ]   [ {Framework} ] - | {Message} |
[----------]   [-------------]   [-------------]   |-----------|

[CATEGORY] [TYPE] [LOCATION] - MESSAGE

 EXAMPLES:

 [IOS] [BREAKING] [FlatList] - Change a thing that breaks other things
 [ANDROID] [BUGFIX] [TextInput] - Did a thing to TextInput
 [CLI] [FEATURE] [local-cli/info/info.js] - CLI easier to do things with
 [DOCS] [BUGFIX] [GettingStarted.md] - Accidentally a thing/word
 [GENERAL] [ENHANCEMENT] [Yoga] - Added new yoga thing/position
 [INTERNAL] [FEATURE] [./scripts] - Added thing to script that nobody will see
-->
Closes #17851

Differential Revision: D6925711

Pulled By: hramos

fbshipit-source-id: 6d53ad2dbed2dca20cd21e5b1b0578be13a91aad

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

@@ -309,7 +309,10 @@ public String getReturnKeyType() {
 
   /*package*/ void commitStagedInputType() {
     if (getInputType() != mStagedInputType) {
+      int selectionStart = getSelectionStart();
+      int selectionEnd = getSelectionEnd();
       setInputType(mStagedInputType);
+      setSelection(selectionStart, selectionEnd);
     }
   }