Add GraphicBufferMapper::preloadHal

Plumbing to allow Zygote to explicitly preload the a.h.graphics.mapper
HAL. An earlier attempt did this implicitly with a static initializer in
libui.so, but the library is also used by non-Zygote processes that
don't have access to HALs. This way, the HIDL libraries and HAL are only
loaded in Zygote and its children, not random other processes.

Bug: 62353585
Test: check that a.h.graphics.mapper is in /proc/`pid zygote64`/maps
Change-Id: I4989c8089e3a4cfd627f8940f5552950c1c617fc
(cherry picked from commit 5dac7819)