Refactor TextLine class

Did following refactoring:
- The surrogate composition is not necessary since this method is only
  interested in TAB character.
- Move BiDi information interpretation logic to Layout.Directions.
- The short circuit for DIRS_ALL_LEFT_TO_RIGHT/DIRS_ALL_RIGHT_TO_LEFT
  is not necessary.
- Renamed the variables.

Bug: 119312268
Test: atest TextLineTest
Test: TreeHugger
Test: Verified locally this doesn't cause performance regression
Change-Id: If987387673ec5a22f703952dfbe45b084664ee59