ART: Presize output vector in RecentAllocations

Reserve the needed storage for each allocation record before dumping the
data to avoid incremental increases.

Effect on b/37620770 repro case: Dump time ~ 12s

(cherry picked from commit d0fc768d)

Bug: 37620770
Test: m test-art-host
Merged-In: I24f46786c9cf15e6523389c630952eff0981f4dc
Change-Id: I24f46786c9cf15e6523389c630952eff0981f4dc