Fix comments with continuation

Backgound: aog/919954 tried to handle
```
   second line
```

but did it incorrectly. The parser works correctly (so this change
reverts aog/919954), it returns multiline comment, but the serializer
converting the internal representation to Blueprint was not emitting
'//' on the lines after the first.

Test: treehugger
Bug: 127521510
Change-Id: I0257a8b3cc4ffcaa6bea44113ceba66bb99d7e43