Only preload egl context

  - The buffer allocator initialization is removed from this flag
as it is caused a memory regression b/384876428
on activities without ui.
  - This change allows us to claw back some of the improvement
and we can follow up with further optimizations afterwards

impact: improves app launch latency in low core devices
  - ~20ms in our view baed app launch
  - up to ~60ms improvements in material3 compose app
  - net neutral memory change

Test: build and app launch test
Bug: 383612849
Flag: com.android.graphics.hwui.flags.early_preload_gl_context
Change-Id: I3009dcf7f9ec86e908cd65239974be208ad6b918