The logic to get the URL for an icon was duplicated in the
EmailFormatter and BasicFormatter. It is now in the abstract
NotificationFormatter, which EmailFormatter and BasicFormatter now
use.
Changes in logic:
* Throw an exception if an invalid notification type is provided instead
of a PHP notice
* icons using 'url' may have different ltr/rtl icons
* Throw exception if icon is supposed to have different icons for
ltr/rtl, but doesn't, instead of debug logging
The new function is static so it can be used in EmailFormatter as it
does not inherit from NotificationFormatter.
Bug: T60726
Change-Id: Ia3c01c35f58eed8cc2c039249ab1ec1a80a8abbb
Instead, have subclasses implement checking required parameters
manually.
The only subclass that was using this was EchoBasicFormatter and has
been updated.
Change-Id: I23e2fa7044e0d59125530024f8c6c35516d3b90b
