preload Buffer Allocator instance on the ViewRootImpl init

- Improves time to first draw by ~5-10ms on low core devices. Earlier
   initialization results in less time during choreographer's first
   frame draw waiting for buffer allocator to be initialized.

Test: Build and wear hermetic app launch test
Bug: 389908734
Flag: com.android.graphics.hwui.flags.early_preinit_buffer_allocator
Change-Id: I41883e735557f186f4ed5004b092f018b7b67574