ultrahdr: asan fix, default initialize member variable

The default value of mExifPos is commented as -1, but this is missing.
Further size_t is usually defined as unsigned int resulting negative
values as overflow. Best to declare it as ssize_t

Bug: 299596513
Test: atest ultrahdr_unit_test

Change-Id: I187f21aea44914993872def5a4889435d3f2d402