Fixes part of: #909

Changes reside in gutenberg PR: WordPress/gutenberg#15122

To test (iOS):

Activate VoiceOver: https://github.com/wordpress-mobile/WordPress-iOS/wiki/Using-VoiceOver

To test (Android):

Activate TalkBack: https://www.wikihow.com/Enable-TalkBack-on-Your-Android
In my case, the system had a handy tutorial on how to use it.
If your version doesn't have it, it's actually very similar to VoiceOver.

Test Steps

Start with selecting the image block and then

• Tap Edit button on toolbar and make sure it has proper title
• Tap Settings button on image toolbar and make sure it has proper title
• Tap Empty Caption and make sure it tells you 'Image caption. Empty'
• Tap Filled Caption and make sure it tells you 'Image caption. '
• When you double tap to the caption you should be able to edit
• Tap on the image and make sure it tells you 'Image. '