Remove unnecessary explicit GC in ZygoteInit.

Removes > 1s of GC time and hundreds of GC during device boot.

Zygote size before: 8388608
Zygote size after: 8388608

No increase in zygote space size due to zygote compaction.

Bug: 16398684

Change-Id: I3c6f5a0b26a207ab39bb0a952400d7488e42f00c