Load the gralloc module at library load

In the a.h.graphics.mapper default implementation, load the gralloc
module in a static constructor that runs when the library is loaded.
This causes Zygote to load the gralloc module and its dependencies
when it loads a.h.graphics.mapper, so that it doesn't have to be
loaded during each activity launch.

Bug: 62353585
Test: boot to launcher, run calculator, confirm gralloc library is in
      /proc/`pid zygote64`/maps

Change-Id: I56defd76b0ba0fba71dea48e2b0d338511563d1f