Boost Zygote to fifo for top-app forks when using fifo ui

When forking for a top app, if the system is using fifo ui, momentarily boost Zygote to fifo priority, execute the fork and reset only the Zygote's scheduling policy back to SCHED_OTHER. This way, we accelerate the cold top-app launches by scheduling the app's main thread at the top app priority sooner by tens to hundreds of milliseconds.

Tested using Perfetto tracing.

Flag: com.android.internal.os.zygote_early_fifo_boost
Bug: 397918406
Change-Id: Ic1113bc72da78af3192b1fc83fb6c4ec893e4792