[LeAudio] Stop initialzing if stopped

There was missing return in LeAudioService#init()

nativInterface could be null when LeAudioService is stopped
right after it's started (e.g., turning on/off BT rapidly).

An additional test ensures the behaivor.
(i.e. it failed withour return)

Tag: #feature
Bug: 237321622
Test: atest BluetoothInstrumentationTests

Change-Id: I0cd0421975926895f2e2a508b848a97e3c327e6e