[audio sharing] Update StreamMetadata element name

Update the StreamMetadata element name to "PM" instead of "MD", as
it was changed in the spec.
https://www.bluetooth.com/specifications/specs/broadcast-audio-uniform-resource-identifier/

Bug: 324655158
Test: atest com.android.settingslib.bluetooth.BluetoothLeBroadcastMetadataExtTest
Flag: com.android.settingslib.flags.enable_le_audio_sharing
Change-Id: I08620bbb209591e70d0d070e1b18b2d2589360db