Increase test coverage in Display area

[JakubVanek]

Current unit testing solution for the display stack is inadequate. A revision of the current testing code is necessary.

[jabrena]

Yep, it is something missing in the library. When we added the nice EV3 Display support, we forget adding it.

Currently, we have this status:

• ev3dev.actuators
• ev3dev.hardware.display
• ev3dev.utils.io
• ev3dev.hardware.display.spi

• LCDStretch
• LCD
• LCDStretch.Flusher

With that contribution, we could enable again the following configuration to reach 80% of code coverage:

jacocoTestCoverageVerification {
    violationRules {
        rule {
            limit {
                //minimum = 0.75 // implement more tests
                minimum = 0.2
            }
        }
    }
}

Related issue: #726

[JakubVanek]

Given that LCDJessie is implemented as a separate class, it seems that the BitFramebuffer & FramebufferProvider paths are mostly redundant. Removing them might reduce the amount of code that needs testing.

[jabrena]

Create a PR with this small change. I will merge directly to master.