Minor fixes on android::base::expected

This is a follow-up change of 7d89fb16.

- Some of the missing conditions for SFNAIE were added
- Fixed indentation
- The assignment operator became "= default"

Bug: 132145659
Test: libbase_test

Change-Id: Ib5232a6e5e1d3df67e185d6e8c03374105c1ce94