Define generic system image for Android targets

generic system image includes system files for all Android targets
and can be used to define the system image.

Bug: 365670526
Test: lunch aosp_cf_x86_64_phone_soong_system-trunk_staging-userdebug
      && m
Change-Id: Ibadd4d95f3224868f46e463b1570e9eb26ce351e