Incorrect line number when using Slf4jLoggerBackend

Hi,

I'm using Flogger with slf4j+log4j2 backend and a pattern that includes the line number (%line).
The line number in the log message does not match my code but instead matches the log statements inside Slf4jLoggerBackend https://github.com/google/flogger/blob/2a6702386859e94c274c79dfad5830d36455d25b/slf4j/src/main/java/com/google/common/flogger/backend/slf4j/Slf4jLoggerBackend.java#L196

Attaching a small project that reproduces the issue.

[line-number-test.zip](https://github.com/google/flogger/files/4344159/line-number-test.zip)

Output:
```
2020-03-17T17:10:53,709 [main] INFO example.LogTest:208 - Info
2020-03-17T17:10:53,712 [main] WARN example.LogTest:211 - Warn
2020-03-17T17:10:53,712 [main] ERROR example.LogTest:214 - Severe
```

The expectation is that the line number will match the log statement in the LogTest. Accurate line number is very important in our log messages.

Thanks.