Support to provide emulator build target for remote goldfish instance

Insert emulator build target into GCE metadata for fetch_cvd to download
emulator images.

Bug: 235388356
Test: acloud-dev create --avd-type goldfish --build-target \
sdk_gphone_x86_64-userdebug --branch git_master

Change-Id: Ie25c75e40de53ad4e13ec4ab49f7cf46ed5f32c6
4 files changed