Fixed race condition while powering down

Made sure powerOffRadioSafely is also run in the main thread
so that it can access DataNetworkController without synchronization.

Fix: 378616116
Test: atest FrameworksTelephonyTests
Test: Manuall power down the device on Pixel 7
Flag: com.android.internal.telephony.flags.power_down_race_fix
Change-Id: Ic645fb9cb324a31274e9dce4a6797694ce996cc8