Move ro.com.android.dataroaming sysprop to aosp_product.mk

aosp_${device}.mk all have this system property set inside
vendor/build.prop:
  ro.com.android.dataroaming=true

so move this common setting into aosp_product.mk

We don't move this to //device/google/${device}/device-${device}.mk
because this property is only used by AOSP builds, so it may cause
issues if we make it a common setting across all device builds.

Bug: 136525499
Bug: 135502030
Test: Compare $ANDROID_PRODUCT_OUT before and after this change
Change-Id: Idd9e235707d9a7fea28fa8c741ad1a4014f71c86