Implement OLED::drawUnavailableIcon() to simplify duplicated calls

[ia]

• Please check if the PR fulfills these requirements

• The changes have been tested locally
• There are no breaking changes

• What kind of change does this PR introduce?
  Implement OLED::drawUnavailableIcon() to simplify duplicated calls.

• What is the current behavior?
  Multiple calls of OLED::drawArea(OLED_WIDTH - 16 - 2, 0, 16, 16, UnavailableIcon) in settingsGUI.cpp with magic constants.

• What is the new behavior (if this is a feature change)?
  Simply call OLED::drawUnavailableIcon() to draw UnavailableIcon which helps:

  • hide magic constants under the hood;
  • remove duplicated calls;
  • simplify future updates/extends for related call in the case of need by changing logic in one single place.

• Other information:
  Check this another one small trivial refactoring out. ;)
  I'm not sure about the necessity of inline but I just wanted to give compilers explicit directive to make calls in place to make sure.

[discip]

Thank you!
Looks good to me. 😊