Alert SoftButton images should be supported by default

[joeljfischer]

Bug Summary

Capabilities, especially images, should be supported by default because some head units don't properly report their image support. If images wind up not being supported, setting the image should be ignored by the head unit.

Reproduction Steps

No known head units fail to support this. This is for alignment purposes (smartdevicelink/sdl_java_suite#1800, smartdevicelink/sdl_javascript_suite#526).

1. Connect to a head unit that supports alert soft button images but doesn't report whether or not they do in display capabilities.

Expected Behavior:
Alert soft button images are shown

Observed Behavior:
Alert soft button images are not shown

iOS Version(s)

n/a

sdl_ios Version

7.3.2-develop

Testing Environment(s)

Unknown

Relevant log output

No response