Allow specifying a network for SNTP time sync

Permits syncing over a specific network instead of the default for
the process. This was causing an issue with Android Wear devices
paired with iOS where the default network is bluetooth
(see b/32663274).

This CL is in support of ag/3776564

Bug: 32663274

Test: adb shell am instrument -e class android.net.SntpClientTest -w \
com.android.frameworks.coretests/android.support.test.runner.AndroidJUnitRunner

Change-Id: Ic9fc169cf75457810d4992121d85d7642e350b90