Add Save button to long screenshot

- Change close to Cancel
- Move buttons to the top
- Fix the edit intent to allow writing
- Boost bottom margin for screenshot image to avoid conflict with
  overview gesture (may want to revisit placement)

Very roughly approximating https://docs.google.com/presentation/d/10i1P2Azxj9S8db1Yc8-9gN7Ruk3sN3gw0XvB6mZUjAU/edit#slide=id.gabe59332cc_11_17

Bug: 179258296
Bug: 179175119
Test: Trigger long screenshot, press save or cancel and verify whether
      image was saved.
Change-Id: I16e9e3d79118a6c23584675834433b383613a199