Allow dynamically shutdown radio and turn it back on

1. Reset the flag mDeviceShuttingDown after radio becomes unavailable
2. Allow setting radio power to ON when current radio power is
RADIO_POWER_UNAVAILABLE

Bug: 416563298
Test: atest
Manual test: with simulator, request to shutdown radio then set radio power to ON
Flag: com.android.internal.telephony.flags.dynamic_modem_shutdown

Change-Id: I046d8ba33549029bcc2b3e237b13df7a4b198121