Update image caption tagname from p to figcaption

This change brings mobile's handling of image captinos in line
with the web. It also addresses a crash that was occurring when
the enter key was tapped to enter a new line in an image caption.

packages/block-library/src/image/edit.native.js

@@ -425,7 +425,7 @@ class ImageEdit extends React.Component {
 								setRef={ ( ref ) => {
 									this._caption = ref;
 								} }
-								tagName="p"
+								tagName="figcaption"
 								placeholder={ __( 'Write caption…' ) }
 								value={ caption }
 								onChange={ ( newCaption ) => setAttributes( { caption: newCaption } ) }