libgav1: import core library snapshot

snapshot @249782802; core decoder only, tests, examples and build files
dropped to reduce the import size

Bug: 131114871
Bug: 130249450
Tested: aosp_arm-eng aosp_arm64-eng aosp_x86-eng aosp_x86_64-eng build

Change-Id: Ic42d6c52ed74554b9437f718735ac356af337345
147 files changed