system/rust: Update GATT datastore interfaces to use raw values

The code almost never makes use of the structured type and mostly relies
on AttAttributeDataChild::RawData.
Removing this type will help convert the pdl rust backend later as the
generated type AttAttributeDataChild will no longer exist under this
form.

Bug: 331817295
Test: m com.android.btservices
Test: atest --host libbluetooth_core_rs_test
Flag: EXEMPT, mechanical refactor
Change-Id: I429341ed0ccc4a8cdaba7cc86d1249f16f11a28e