Reland: Add PRODUCT_INSTALL_DEBUG_POLICY_TO_SYSTEM_EXT

If this option is set, then an additional copy of the debug policy can
be installed to the GSI, and the init-second-stage of GSI could load
debug policy from GSI /system_ext when debug-ramdisk is used.

Bug: 188067818
Test: Flash RQ2A.201207.001 bramble-user with debug ramdisk & flash
  gsi_arm64-user from master, device can boot and `adb root` works
Change-Id: I8c62a3cea026bd26b1994092a14238d22ba1e2df