companion: Fix credentials not saved

[postlund]

Relates to #2216

[codecov[bot]]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (1c4b4d3) 88.96% compared to head (c281b39) 88.94%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2217 +/- ## ========================================== - Coverage 88.96% 88.94% -0.02% ========================================== Files 167 167 Lines 11347 11354 +7 ========================================== + Hits 10095 10099 +4 - Misses 1252 1255 +3 ``` | [Files](https://app.codecov.io/gh/postlund/pyatv/pull/2217?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Pierre+St%C3%A5hl) | Coverage Δ | | |---|---|---| | [pyatv/protocols/companion/pairing.py](https://app.codecov.io/gh/postlund/pyatv/pull/2217?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Pierre+St%C3%A5hl#diff-cHlhdHYvcHJvdG9jb2xzL2NvbXBhbmlvbi9wYWlyaW5nLnB5) | `100.00% <100.00%> (ø)` | | | [pyatv/protocols/mrp/protobuf/\_\_init\_\_.py](https://app.codecov.io/gh/postlund/pyatv/pull/2217?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Pierre+St%C3%A5hl#diff-cHlhdHYvcHJvdG9jb2xzL21ycC9wcm90b2J1Zi9fX2luaXRfXy5weQ==) | `99.57% <100.00%> (+<0.01%)` | :arrow_up: | | [...tv/protocols/mrp/protobuf/DeviceInfoMessage\_pb2.py](https://app.codecov.io/gh/postlund/pyatv/pull/2217?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Pierre+St%C3%A5hl#diff-cHlhdHYvcHJvdG9jb2xzL21ycC9wcm90b2J1Zi9EZXZpY2VJbmZvTWVzc2FnZV9wYjIucHk=) | `61.90% <14.28%> (-14.57%)` | :arrow_down: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/postlund/pyatv/pull/2217/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Pierre+St%C3%A5hl)

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