LeAudioTestApp: Fix crash issue when starting broadcast

getAllBroadcastMetadata will return immutable Collections.emptyList()
in error cases, this might cause following list add operation in
updateBroadcastMetadata report UnsupportedOperationException.

the fix is to use ArrayList instead if the broadcastMetadata list is
empty.

Bug: 287099760
Test: mma LeAudioTestApp
Test: manual test with LeAudioTestApp
Change-Id: I2c7b0ca8ac2d431030940936f754f822b1cc9779