Dismiss keyboard when non-textual block is selected

[daniloercoli]

This PR fixes #308 by making sure that to call blur on the textual field that lost the focus.

React Native Aztec Wrapper PR is here:wordpress-mobile/react-native-aztec#93

GB side PR: WordPress/gutenberg#12765

Steps to repro:

• Tap on a para block
• Tap on the page break block
• The block holder should be visible on the page break block, and the keyboard dismissed
• Repeat the steps above by tapping on any of the TextInput "block", and then tap on another kind of block --> the keyboard should be visible only when the focus is on a TextInput field.

Note: The caret is still visible (and blinking) in the TextInput "powered" block that lost the focus. We will fix this in another PR.

[mzorz]

LGTM 👍 remember to update the hashes once Gutenberg-side PR gets merged