Fix stack overflow when log the invalid SignalStrength level

The change fixes the issue by not logging "this" in SignalStrength.getLevel().

SignalStrengthTestable calls SignalStrength.getLevel in its toString(). An invalid SignalStrength level in SignalStrength.getLevel will log "this" which calls SignalStrengthTestable.toString(), causing stack overflow due to cyclic calls.

Bug: 256874469
Test: Manual test for API used via adb shell
Flag: EXEMPT(test only)
Change-Id: Id70275119efc2e9507483f426c7ecc136e6c05f5