This PR fixes an issue that happens when the "middle" button is pressed on a page that should not handle middle or left button presses. If, when releasing the "middle" button (left + right), the left one is the first one to be released, this triggered a left button press which would cancel an otherwise uncancelable page, like this one:

To reproduce the bug (before the fix):

• trezorctl device setup
• keep confirming ever page until you arrive at the page shown in the screenshot above
• press the left button and keep it pressed
• while the left button is pressed, press the right button and keep both pressed
• release the left button
• now the process is canceled even though it should not be possible to cancel it

Note: this fix affects both the ButtonController and the ButtonPage. Normally one of the two would be enough to fix the issue and this fix indicates that there is some redundancy in the logic that could be perhaps refactored.