Add an entry in METADATA.txt to determine whether to use the CMC GC.

If the device uses the CMC GC, the file will contain:
extra-args = --runtime-arg -Xgc:CMC

Otherwise, the file will contain an empty line:
extra-args =

Bug: 287652269
Test: -
  1. m dist out/dist/boot.zip
  2. unzip -p out/dist/boot.zip METADATA.txt
Change-Id: I3d50367eb2a341e1c03286d7a2cd7ed44d59708f