Add toString() method to PhysicalChannelConfig

We need to be able to print the PhysicalChannelConfig
for debugging/dumping, so adding a toString() method
to print in a format that we can easily digest and
is consistent with other Telephony log formatting.

Bug: 78791811
Test: manual / TelephonyDebugMenu
Change-Id: Ieb12f78a821369072ca9f03d28b28759836f84b4