BREAKING - default underlineColorAndroid to transparent

Summary:
Set default `underlineColorAndroid` to `transparent`.
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Fixes #18938

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Use a TextInput in a component without defining `underlineColorAndroid`, the underline color should be transparent.

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[ANDROID] [BREAKING] [TextInput] - set default underlineColorAndroid to transparent

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  **INTERNAL and MINOR tagged notes will not be included in the next version's final release notes.**

    CATEGORY
  [----------]      TYPE
  [ CLI      ] [-------------]    LOCATION
  [ DOCS     ] [ BREAKING    ] [-------------]
  [ GENERAL  ] [ BUGFIX      ] [ {Component} ]
  [ INTERNAL ] [ ENHANCEMENT ] [ {Filename}  ]
  [ IOS      ] [ FEATURE     ] [ {Directory} ]   |-----------|
  [ ANDROID  ] [ MINOR       ] [ {Framework} ] - | {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 https://github.com/facebook/react-native/pull/18988

Reviewed By: mdvacca

Differential Revision: D7765569

Pulled By: yungsters

fbshipit-source-id: f7ad57a46fc0d18b47271ca39faae8c635995fbb

Libraries/Components/TextInput/TextInput.js

@@ -637,6 +637,7 @@ const TextInput = createReactClass({
   getDefaultProps(): Object {
     return {
       allowFontScaling: true,
+      underlineColorAndroid: 'transparent',
     };
   },
   /**