[RNTester] Fix tabIndex Example

[agniuks]

In order to test tabIndex, a component must be focusable. In the keyboarding example page we use TouchableHighlight in the tabIndex example, this adds an onPress() function to those components to make them focusable and make the example functional.

Microsoft Reviewers: Open in CodeFlow

[NickGerleman]

Consider explicitly setting "focusable".

History here is we used to have a prop "acceptsKeyboardFocus", and our own logic for whether a Touchable should be focusable by default. A focusable prop, with its own logic, was added upstream. This added the behavior that disabled, or Touchables missing callbacks are not focusable by default.

[agniuks]

Looks like the checks are failing in TextViewManager, likely #8454 conflicting with other changes to that file since the checks on it last ran - working on a fix.