Add Attribute response for GetElementAttributes

When retrieving all Attributes from GetElementAttributes
we have to respond with all Attributes, even those for
which we don't have a value.

Bug: 346721681
Flag: com.android.bluetooth.flags.get_all_element_attributes_empty
Test: atest BluetoothInstrumentationTests
Change-Id: I24830c86471a5b0bfe818389bd5e3d57ea145c64