Ensuring thread-safe usage of DateFormat.

DateFormat is inherently unsafe for multithreaded use. This patch adds
proper synchronization. The 'sLock' is re-used. To avoid two consecutive
lockings, locked version of initFormatStrings() method is created and
used where appropriate, i.e. in original method AND in added synchronized
block.

Change-Id: Id3c34613623e743e703aadb2342aa788679dda8a