Build GSIs (aosp_$arch products) in sparse format

After aosp/719235 is merged, mk_qemu_image.sh now accepts
images in both raw and sparse formats. So We can build GSIs
(system images of aosp_$arch products) in sparse format, which,
historically, is expected by users of GSIs.

Bug: 111775319
Test: The system images below are all in sparse format:
    $ lunch aosp_x86-userdebug; $m -j; emulator
    $ lunch aosp_x86_64-userdebug; $m -j; emulator
    $ lunch aosp_arm-userdebug; $m -j; emulator
    $ lunch aosp_arm64-userdebug; $m -j; emulator

Change-Id: I4cb8df0908c4618376117249b61c853fac947e47