ErrorProne: BluetoothMapContentObserver

Remove a lot of unused code and empty javadoc tag
and the following

```
539|830|2522: warning: Inner class is non-static but does not reference enclosing class
    private class ConvoContactInfo {
            ^
    /*package*/ class Msg {
                ^
    private class PushMsgInfo {
            ^
867: warning: Prefer instanceof to getClass when implementing Object#equals.
        public boolean equals(Object obj) {
                       ^
3180: warning: Date has a bad API that leads to bugs; prefer java.time.Instant or LocalDate.
        msgInfo.timestamp = Calendar.getInstance().getTime().getTime();
                                                                    ^
```

Bug: 236759221
Test: m RUN_ERROR_PRONE=true Bluetooth
Change-Id: I902f51234338217064e241952ab8771d451108e6