ADM: fix active device fallback on handleHfpActiveDeviceChanged

If isDualModeAudioEnabled was false, path of HFP device== null  was mistakenly taken,
and setFallbackDeviceActiveLocked was called. We should only fallback if
device == null AND isDualModeAudioEnabled is false.

Bug: 379236455
Flag: EXEMPT, regression verified with unit test
Test: atest ActiveDeviceManagerTest#switchActiveDeviceFromLeToHfp_noFallbackToLe
Change-Id: Ieeda81694dd171e561764178cb1ed06cf13da30d