Boost process priority during fork.

The zygote process sets MAX priority for the child process after a
fork if it is a 'high priority fork' (e.g. for top-app). However,
the zygote does a fair amount of work before the fork, and can take
up some amount of time due to getting descheduled. This introduces
latency in app-launch, especially on low-power devices.

Fix this by setting the zygote process priority to MAX for a
high-priority fork early on, and reset it back to DEFAULT
afterwards.

Bug: 323891634
Test: manually
Change-Id: Iac1ea81531f59ee6245c73530f2dd639209421f7