ExactCalculator: bring back separate parentheses buttons

A single parenthesis button does not allow to write all possible
valid expressions that the calculator supports.

Example expression that could not be written:
  1*((2+3)*(4+5)+pi)!

To make space for the two buttons, move back the clear button
to the long-press action of the delete button.

Reverts: 9691e2fe
Reverts: 3daf951a

Change-Id: I8fe696e23d75545a1ee23641448ea31ce31da0c0