Prevent deadlock when using always-on legacy VPNs

The following deadlock can happen when using an always-on legacy
VPN connection:

"Binder:7444_7" prio=5 tid=94 Blocked
  at com.android.server.connectivity.Vpn.isBlockingUid(Vpn.java:-1)
  - waiting to lock <0x032141f0> (a com.android.server.connectivity.Vpn) held by thread 52
  at com.android.server.ConnectivityService.isNetworkWithLinkPropertiesBlocked(ConnectivityService.java:1041)
  - locked <0x0791429e> (a android.util.SparseArray)

"ConnectivityServiceThread" prio=5 tid=52 Blocked
  at com.android.server.ConnectivityService.getLinkProperties(ConnectivityService.java:1351)
  - waiting to lock <0x09ec3887> (a com.android.server.connectivity.NetworkAgentInfo) held by thread 103
  at android.net.ConnectivityManager.getLinkProperties(ConnectivityManager.java:1065)
  at com.android.server.connectivity.Vpn$LegacyVpnRunner.<init>(Vpn.java:1524)
  at com.android.server.connectivity.Vpn.startLegacyVpn(Vpn.java:1413)
  - locked <0x032141f0> (a com.android.server.connectivity.Vpn)

"Binder:7444_D" prio=5 tid=103 Blocked
  at com.android.server.ConnectivityService.isNetworkWithLinkPropertiesBlocked(ConnectivityService.java:1039)
  - waiting to lock <0x0791429e> (a android.util.SparseArray) held by thread 94
  at com.android.server.ConnectivityService.reportNetworkConnectivity(ConnectivityService.java:3249)
  - locked <0x09ec3887> (a com.android.server.connectivity.NetworkAgentInfo)

Lock <0x0791429e> before <0x09ec3887> in thread 103 so that thread 94
will lock <0x032141f0> when <0x09ec3887> is not locked, allowing thread
52 to complete.

BUGBASH-1197

Change-Id: Ifaf4d4dc0053dd71ab0734e3f3b1e9839eece22a