Add Legacy GSI R

Legacy GSIs named with the suffix _ab, ex. aosp_arm64_ab.
It contain some backward-compatible configurations for
devices upgraded from O/O-MR1.

The patch also addes legacy_gsi_release.mk for the special
configuretions of Legacy GSI.

Bug: 144742991
Test: boot aosp_arm64_ab-userdebug on a walleye P (with SPL)
Test: boot aosp_arm64_ab-userdebug on a walleye Q (with SPL)
Change-Id: I61c9e74abcc921ab2cb50bf6f6dce1097a8d6c77