build: Add chromium prebuilt support (1/2)

Ported from https://github.com/MoKee/android_build/commit/ee7cc925d405815db3d2a712941e3213f308ea1e
export USE_PREBUILT_CHROMIUM=1 to enable

Change-Id: I2478d83e6bc4ba60971a85db549095f7bd005139