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: I9c3dad8bb6c5fa88b16762193446dc7e54f326c8
Merged-In: I9c3dad8bb6c5fa88b16762193446dc7e54f326c8
(cherry picked from commit 0a468fe9)