Layout and style updates for LongScreenshotActivity

- Give save button accent color.
- Remove cancel button (may come back in another form later)
- Share button becomes and icon on the right.
- Edit button moves to the bottom, gets icon + text

Bug: 180967468
Test: Visual eval with light and dark themes
Change-Id: I19ccea06e7dbec47b26ffd0684ebb7e31bdfe8b4