SystemServer: Move sensor init to a new thread.

Move Sensor init to a separate thread so that it doesn't block the
critical booting path.

Initializing Sensors in separate thread has been seen to improve boot
time on customer devices with low-tier / ult chipsets(less cpu cores)

Change-Id: I5c46a5805c04ffe551019b4683523e3bf70274a2
Framework/base: Relocating GC calls at the end of loop during zygote init

Relocating explicit GC calls at the end of the loop during the preloading
the resources and preloading clases, with this changes boottime improved.

Change-Id: I2e05ef68578eddca8cd435b5186e198ee7d71739