Fix race in TelephonyScanManager

TelephonyScanManager may sometimes encounter a race condition where
Telephony sends it the first scan result before it has saved the
returned scan ID. This causes a crash in the app that's requesting the
scan.

To fix this, synchronize on mScanInfo while requesting the network scan
so that the handler in TelephonyScanManager will wait for the scan ID to
be saved before processing the first message from Telephony.

Bug: 142068098
Test: manual
Change-Id: I0b2b4c6dc749e21641843818293941704ed870a6