[RNMobile] Tiled Gallery Block: Update placeholder text

[SiobhyB]

Fixes wordpress-mobile/gutenberg-mobile#4195

With this PR, the placeholder text is changed from "ADD IMAGE" to "ADD MEDIA". This is to bring consistency with the text that's seen in other, similar blocks, such as the Gallery and Story blocks.

Gutenberg Mobile PR: wordpress-mobile/gutenberg-mobile#4194

Changes proposed in this Pull Request:

• The MediaPlaceholder component's instructions prop is used to update the block's default placeholder text. This matches up with how the placeholder text is set here in the Story block.

Jetpack product discussion

Main project thread for the Tiled Gallery block on mobile: p9ugOq-1Tb-p2

Does this pull request change what data or activity we track or use?

No, it doesn't.

Testing instructions:

With these branches checked out and the Gutenberg Mobile demo app running, add the Tiled Gallery block. The placeholder text can be found within the default, empty block. It should be verified that the text reads "ADD MEDIA".

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[guarani]

I manually added the fix from #21625 in order to test this and it looks great on both platforms 🙇