[link-quality] track "link quality out" in LinkQualityInfo (#11155)

This commit updates `LinkQualityInfo` to track the "link quality out,"
which is the link quality from the neighbor's perspective. This value
is currently tracked by the `Router` class. The existing methods in
`LinkQualityInfo` that return the "link quality in" are renamed to
use `LinkQualityIn` to clarify and distinguish them.

This change uses existing bits in `LinkQualityInfo`, freeing up bit
fields in the `Router` class to be used for other information.

tests/unit/test_link_quality.cpp

@@ -114,7 +114,7 @@ void TestLinkQualityData(RssTestData aRssData)
         PrintOutcome(linkInfo);
     }
 
-    VerifyOrQuit(linkInfo.GetLinkQuality() == aRssData.mExpectedLinkQuality);
+    VerifyOrQuit(linkInfo.GetLinkQualityIn() == aRssData.mExpectedLinkQuality);
 }
 
 // Check and verify the raw average RSS value to match the value from GetAverage().