PhoneConfigManager: server-side flag for vDSDA

- PCM reads Telephony property 'enable_virtual_dsda' thru DeviceConfig
  and updates staticPhoneCapability.
- PCM no longer reads device overlay flag 'config_enable_virtual_dsda'
  at RADIO_ON, which is deprecated and will be removed.
- 1p: go/vdsda-experiments

Bug:27727750
Test: Manual regression, need to verify DSDA works after server-side
feature flag update.

Change-Id: Idbdfcb566af7466771b4f3caad3faa409b326273