将MaterialToolbar的标题改为中心

<com.google.android.material.appbar.AppBarLayout
    ...
    >

    <com.google.android.material.appbar.MaterialToolbar
        ...
        app:contentInsetStartWithNavigation="0dp"
        app:contentInsetStart="0dp"
        android:gravity="center"
        />

</com.google.android.material.appbar.AppBarLayout>

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    MaterialToolbar toolbar = findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    getSupportActionBar().setTitle("");
}