[le audio] remove the ending null bytes in native broadcast code

While createBroadcast using broadcast code setting from UI, JNI
layer will add null bytes padding on MSB positions if code is shorter
than 16.

After broadcast is started successfully and prepare to notify UI via
onBroadcastMetadataChanged() callback, JNI layer need remove the
ending null bytes from the native broadcast code.

Bug: 301913633
Bug: 316005152
Test: manual
Test: atest LeAudioBroadcastServiceTest
Change-Id: I734110bb6f16c63e46ff44c91bc6b7eb29241b97