Fix inconsistency in LoggingPrintStream.

LoggingPrintStream prints "null" for null strings from
print(String) but throws an NPE from println(String).

Fix this by delegating to the underlying builder (and avoiding
the "fast" path when the input String == null)

bug: https://code.google.com/p/android/issues/detail?id=63380
Change-Id: I89c5c12db6ed5af8037d46f67369b38c5b4cbc9f