[LeBroadcast] Fix the NullPointerException for launching dialog

Since the BluetoothLeBroadcastMetadata is null, and it causes the
NullPointerException.
After the system starts broadcast, the framework sends two
callbacks, first is onBroadcastStarted, and second one is
onBroadcastMetadataChanged. The BluetoothLeBroadcastMetadata object
is created before onBroadcastMetadataChanged. Therefore,the app
needs to wait the onBroadcastMetadataChanged, and then launchs the
broadcast dialog. If the app launchs after the onBroadcastStarted,
then the broadcast dialog can't get the null
BluetoothLeBroadcastMetadata and it causes UI show the wrong QR code.

Bug: 237976782
Test: build pass and manually test

Change-Id: I91a28079ae01bbc5cc4c3ce9331741f0b4ee81c0