Rename sharesheet started atom proto fields.

When copying over the fields into Google3 the linter failed on two field
names that are camel case rather than snake case. Changing here so the
two match. In addition, one of the enum fields didn't have a default
value.

Bug: 147508495
Test: compiles + ChooserActivityTest runs
Change-Id: Iacec9654350763e691a4a10087f693839fd5ea83