Don't try to handle callbacks until mSystemReady is set.

Bug: 160855861
Bug: 161547406
Test: atest ./hostsidetests/net/src/com/android/cts/net/HostsideRestrictBackgroundNetworkTests.java
Change-Id: Icf570122cae0d9b03319d16df913904bab8ca18d
1 file changed