Make sure Zygote is running at process priority 0 after VM has started.

Boosting up zygote priority before VM startup, saves ~450ms of boot time
for N9, 180ms for Nexus 5X.

(cherry picked from commit 5d551a5a)

Bug: 28866384
Test: m
Test: Device boots
Change-Id: Ic85892b408e15bbc1de7ce706f113f23974fe478