Represent OpenFormula as list of formulas

Use list of formulas inside OpenFormula, rather than using an exact 2
formulas. The list of formulas would help in representing multiple
formulas with the same connector, which would create less nesting.

Bug: 141979167
Test: atest FrameworksServicesTests:OpenFormulaTest
Change-Id: I8938c842dfe22c21bb8b4dd48e7ce2f639f33854