Change emulator system image to unsparse format

Currently, emulator system image is the non-legasy GSI, which is in sparse
format. Several internal clients don't support sparse format, and other
internal/external clients usually support both. The reason to choose sparse
format for GSI may not be valid any more. See bug for detail.

Bug: 130341962
Test: Checked the header of system.img to see if it's unsparse.
      Boot emulator aosp_x86 successfully.
      Boot mixed GSI and CF x86 successfully.

Change-Id: I839f49809429227607b2e876a4a3628df43a97b4