Show "add block here" placeholder when adding block from title

[mchowning]

Summary

Fixes #632 : "Add Block Here" (ABH) indicator isn't shown when title field is focused.

Related gutenberg PR

Before	After

Remaining Issue

The ABH indicator is still not shown when a new post is created and a new block is added before the user has interacted with the post content (i.e. if the user either tries to add a block first thing or only interacts with the post title, then adding a new block will not display the ABH indicator). This is because the ABH indicator is attached to our block holders, but until the user taps on the "Start writing..." placeholder text in a new post, the post does not actually contain any blocks (it just has a RichText field that is replaced with a block onTouch). This actually may not be a bad thing since in that scenario we replace the entire "Start writing..." text with whatever block is added, but it is inconsistent with how we handle other similar scenarios. I will reach have reached out for some feedback in the issue and will make any needed updates to address this edge case in a separate PR.

To Test

Main Functionality

1. Open demo app (or if using the full app, open a post that is not empty in order to avoid ⬆️Remaining Issue⬆️)
2. Tap on the post's title
3. Tap the ➕ to bring up the add post bottom sheet
4. Observe the ABH indicator is shown immediately below the title and above the first block in the post

Regression check

1. Open demo app (or if using the full app, open a post that is not empty in order to avoid ⬆️Remaining Issue⬆️)
2. Add an empty paragraph block
3. Insure that new empty paragraph block is selected
4. Tap the ➕ to bring up the add post bottom sheet
5. Observe that the ABH indicator appears above the selected empty paragraph block
6. Dismiss the add post bottom sheet
7. Add some content to the paragraph block so that it is not empty
8. Tap the ➕ to bring up the add post bottom sheet
9. Observe that the ABH indicator appears below the selected non-empty paragraph block

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.

[marecar3]

Nice work @mchowning ! 🎉
I think that we are good to go with this one!