This PR moves accessibility handling of unselected blocks to block-holder, so:

• We don't have to handle blocks separately. Every block will have at least their name(including unknown blocks) read by screen reader without extra coding.
• If we want to provide more information for a block we'll need to inject its own getAccessibilityContent function as its attribute.
• Some block content like Separator block have very little height, it is hard to select them otherwise.

gutenberg PR: WordPress/gutenberg#15214

Note: This is still a kind of experiment, we can decide not to merge this based on your opinions and tests.

To Test:

It should be enough to test in only 1 platform.
Check below items with Voice Over on:

• Image block empty
• Image block with image - selected
• Image block with image - unselected
• Next page
• Paragraph block empty
• Paragraph block with text
• Post title empty
• Post title with text
• Unrecognized block
• Separator block (Should tell its name)
• Basic operations like Updating/Closing/Opening post