Init EGL only for HW-accelerated Activities

Most processes might end up using  EGL, but most don't except for
Activities with HW-accelerated UI. And for other cases, the startup
latency of initializing EGL on-demand isn't as important. So this
change only tries to early-initialize EGL for HWUI Activities.

It also fixes a logic problem that may or not have been an actual bug:
previously, we only chose a graphics driver if we successfully set up
shader cache directories, even though these are mostly unrelated. Now
we always choose a graphics driver, except in isolated processes that
can't use graphics.

Bug: 38215658
Test: systrace framework start and Clock launch, check eglGetDisplay
      is called by RenderThread in non-HWUI-Activity cases, and is
      called on a separate thread before RenderThread needs it for
      HWUI-Activity cases.
Change-Id: I101e5578a9d7c508d232d0edeed7ceff9d8a74d6