Refactor onBackPressed to make it easier to read

Bug: 30944495

Test: manual
- press Back button in:
  - All contacts view, and in search/selection mode.
  - Duplicates view
  - Label view, and in selection mode.
  - Account view, and in search/selection mode.
  - No contacts view.

Change-Id: Ie6ef9c5c2b701b3d8e7f97f1bba31cb247661d30