libhwui: avoid creating GPU context in Zygote

Creating GPU context besides loading the GPU driver would potentially
affect graphics driver updatability. Currently, we always preload
graphics drivers in Zygote to speed up app startup time. Upon using
updatable driver, we'll unload the current driver and load the uploaded
one. Although the symbol table is fully protected by the linkerconfig we
setup in Android 10, we still have to make sure the driver has a clean
state upon unloading and reloading.

At this moment, Vulkan driver unloading has not been supported yet. So
if we are going to turn on skia Vulkan backend by default, we should
avoid preloading the Vulkan driver until the unloading feature is
supported for Vulkan.

Bug: 162242036
Bug: 135536511
Test: atest CtsUiRenderingTestCases all pass on GL backend
Test: atest CtsUiRenderingTestCases no regression on VK backend
Change-Id: I9084586315a7593be2cfe17710620aacce568d36