Fix temporary file name strlcpy size.

The strlcpy size argument overflows on integer sanitized builds.

 runtime error: unsigned integer overflow: 11 - 35 cannot be represented
 in type 'unsigned long'

This doesn't cause a problem because strlcpy stops on the null in
TMP_FILE_PATTERN, and localTmpFileName is defined large enough to
contain TMP_FILE_PATTERN.

This changes the size argument to the remaining length in the buffer,
and removes an extraneous '/' write as TMP_FILE_PATTERN already begins
with '/'.

Bug: 30969751
Test: Reran CTS test which triggered the overflow.

Change-Id: I20b5deeaaa1a863324dfd1d94f3135920eae48d3