ChangeScreen2: Scroll to make last viewed file visible

When navigating up from a file view back to the change, scroll to
make the last viewed file visible in the "Files" section of the
screen. This is useful on changes with incredibly large commit
messages like I2d572279ac978e644772b1cedbecc080746a2306. The files
table is immediately visible and the user can navigate to another
file within the change.

Change-Id: I4aa99c1632e60deb7254803f3730df45d6cfc46b
3 files changed