Clarify "preferred audio route" logs.

I have lost count of the number of times that triagers see
"getBaseRoute: preferred audio route is null" and say, "Telecom is setting
the audio route to null, PTAL", when in fact this is just our way of saying
that the AudioManager did not specify a preferred audio route, so Telecom
needs to fall back to determining what it should route to instead.

Flag: NONE logs only change
Test: Logs only change.
Bug: 427977537
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:aab342b88cd304971f3f54e1b2bb23a4c847ec86)
Merged-In: Ia64ed60682300762616e5adb470f93dc7b1ab2eb
Change-Id: Ia64ed60682300762616e5adb470f93dc7b1ab2eb