ahat: fix bug parsing ids with the high bit set.

With this fix, ahat will no longer generate broken links for objects
with ids greater than 0x7FFFFFFF.

Bug: 69706820
Test: m ahat-test
Test: Manually heap dump from b/69706820 and verify ids parsed correctly.
Test: Automated test planned for followup as part of b/69706820.
Change-Id: Ie49f49f8959953cff54a543eb7a25bed806b7f1c