Use GetStringRegion/GetStringUTFRegion where appropriate.

Note that the changes to DecimalFormatInterface.cpp and RBNFInterface.cpp
are just minor tidy-ups, fixing an issue where the early error exit wouldn't
call ReleaseStringChars to undo the earlier call to GetStringChars. Also
remove a dead function and fix a comment in ExpatParser.cpp.

Tested on sapphire-eng.

Bug: 1639287
5 files changed