Feature/add decimal pad to android (#19714)

Summary:
For a current use-case we need the a keyboard with characters 0-9 and a decimal point (or comma depending on language settings)

This exists on iOS as UIKeyboardType "decimalPad" and this is what react-native maps to for both "numeric" and "decimal-pad". This also exists on Android as inputType "numberDecimal", but is currently not accessible through react-native.

This PR maps the value "decimal-pad" of the keyboardType property of TextInput to the Android inputType "numberDecimal", effectively making "decimal-pad" cross platform without breaking anything.

* https://facebook.github.io/react-native/docs/textinput.html#keyboardtype
* https://developer.apple.com/documentation/uikit/uikeyboardtype
* https://developer.android.com/reference/android/widget/TextView#attr_android:inputType

There is this bug in some Samsung keyboards where both the - sign and decimal sign disappear when the keyboardType is set to "number" and both the "signed" and "decimal" flags are set. (Like is the case when using the react-native keyboardType prop "numeric".) https://androidforums.com/threads/numeric-soft-keyboard-missing-minus-sign-in-android-8-0-samsung-a5.1272628/

For developers that need decimal numbers but not negative ones, using "decimal-pad" will provide a workaround. I reproduced this on a Samsung A5 only, but maybe other phones have this exact issue. https://github.com/facebook/react-native/issues/12988 https://github.com/facebook/react-native/issues/12977 https://github.com/facebook/react-native/issues/17473 https://github.com/facebook/react-native/issues/17474

* Added testcase consistent with existing keyboardType tests
* Also added testcase for the related, but missing number-pad

This PR follows the same approach as the recently merged PR introducing "number-pad" b638847a46

Documentation PR: https://github.com/facebook/react-native-website/pull/405

 [ANDROID] [ENHANCEMENT] [TextInput] - Added "decimal-pad" keyboard type
Closes https://github.com/facebook/react-native/pull/19714

Differential Revision: D8429185

Pulled By: mdvacca

fbshipit-source-id: 6b56da2088f2be427ebffa04c4e17c91ffb9f7d9

Libraries/Components/TextInput/TextInput.js

@@ -86,12 +86,12 @@ export type KeyboardType =
   | 'numeric'
   | 'phone-pad'
   | 'number-pad'
+  | 'decimal-pad'
   // iOS-only
   | 'ascii-capable'
   | 'numbers-and-punctuation'
   | 'url'
   | 'name-phone-pad'
-  | 'decimal-pad'
   | 'twitter'
   | 'web-search'
   // Android-only
@@ -388,6 +388,7 @@ const TextInput = createReactClass({
      * - `default`
      * - `numeric`
      * - `number-pad`
+     * - `decimal-pad`
      * - `email-address`
      * - `phone-pad`
      *
@@ -399,7 +400,6 @@ const TextInput = createReactClass({
      * - `numbers-and-punctuation`
      * - `url`
      * - `name-phone-pad`
-     * - `decimal-pad`
      * - `twitter`
      * - `web-search`
      *