Fix documentation bug with METADATA_KEY_LOCATION

MPEG4Writer::writeGeoDataBox() has always written latitude
before longitude since it was introduced. And the ISO-6709
is also to have +-{latitude}+-{longitude}/ so we should have
the correct information here.

This is likely to be important for Q since lat/long are no
longer accessible via MediaStore.

Bug: 134100244
Test: documentation change only
Change-Id: If157e3b0a639da9e012c0dd23fcfacfa0a59c28e