Use framework onNavigateUp() instead of simply finish().

- Simply finishing activity bypasses the entire nav-up chain, making
  taskAffinity/parentActivity defined in androidmanifest.xml no-op, which
  is bad.

- Also removed showMenu() logic, this is from drawer era and was only
  needed by mobile settings. We no longer need drawer so this method can
  be removed.

Bug: 72337421
Test: atest

Change-Id: Idee1b72751363b5162887a799197404a9498c83e