Do not include CellBroadcastLegacy for Go devices

CellBroadcastLegacy app is for migrating data from pre-mainline world.
Go devices do not take mainline module, thus excluding this apk
from build. Otherwise, both CellBroadcastLegacy app and
CellBroadcastAppPlatform will have the same package name
com.android.cellbroadcastreceiver

Bug: 157702986
Test: build go target:  walleye_svelte-userdebug
Change-Id: I254a8af334580b79ffdbc9e8bfa95b23500f507e