Set legacy_android10_support to apex modules

resolv apex should support Android 10.
With this flag, apex packages will include older APEX manifest format
(JSON) as well as new format (Protobuf) so that they will work with
Android 10.

Bug: 143951586
Test: m com.android.resolv
      and install this to Android 10 device
Change-Id: I887e45d28ce3e2c65c8849d9d18bd6489fe23465