Avoid duplicate hash calculation in ImageWriter.

The hash was calculated by both TableSlot constructor
and HashSet<>'s `insert` function.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 175869411
Change-Id: I4eb73bcd0a0d1abc2247f8a3c948ca242201194c
1 file changed