Improved screen reader accessibility of message list in regard to up to date features since this has been looked into last time.
It includes these changes:
* Explicitly set contentDescription of image buttons and image views to @null rather than setting it into textual values where these are not used.
* Some 3rd party screen reader apps don't disable their contentDescription calculation from child widgets while the parent has a custom actions and overridden contentDescriptions so use setImportantForAccessibility on all the widgets contentDescription is populated from. This avoids speaking some texts multiple times on non default accessibility service setups.
* Added missing details to the contentDescription such as signed, notes, labels, keywords.
* Change it so visibility of the individual widgets is respected where possible.
Signed-off-by: Peter Vágner <pvdeejay@gmail.com>
Peter Vágner