Base: Hand complete log message to aborter

Undo zero-termination-substitution for linebreaks when logging.
This results in handing the complete log message to the aborter.
Add a test.

Bug: 31893081
Test: mmma system/core/base && $ANDROID_HOST_OUT/nativetest64/libbase_test/libbase_test64
Change-Id: I2ef6c6351db2fd494a02985f634f439104136227
diff --git a/base/logging.cpp b/base/logging.cpp
index ece10ec..dab86fe 100644
--- a/base/logging.cpp
+++ b/base/logging.cpp
@@ -415,6 +415,8 @@
         msg[nl] = '\0';
         LogLine(data_->GetFile(), data_->GetLineNumber(), data_->GetId(),
                 data_->GetSeverity(), &msg[i]);
+        // Undo the zero-termination so we can give the complete message to the aborter.
+        msg[nl] = '\n';
         i = nl + 1;
       }
     }