bug(table/button): Z-index and scroll issues in a sticky table

### Is this a regression?

- [X] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

14

### Description

I have two bugs with a sticky mat-table and not with table with mat-table directive :

- The button's z-index has changed since version 15.0.0. The sticky and button column are both 1, which makes the button visible (but not the table background), a Workaround st z-index to 2 on sticky column.
- When I have several columns in a table with a fixed column, the fixed column also moves.

### Reproduction

StackBlitz link: https://stackblitz.com/edit/tu9ax3

### Expected Behavior

That anything that scrolls under the sticky column is not visible and the sticky column is real sticky.

### Actual Behavior

The button is above the sticky column, and the sticky column is not really sticky.

### Environment

- Angular: 15 - 16
- CDK/Material: 15 - 16
- Browser(s): Chrome Lastest version
- Operating System (e.g. Windows, macOS, Ubuntu): Windows 11

リンク先の StackBlitz の再現から始め、固定列とボタン列を持つ sticky mat-table について、Angular/CDK/Material 14 と 15 の動作を比較します。スクロールするコンテンツが sticky 列の下に隠れること、固定列が固定されたままであること、ボタンがその上に表示されなくなっていることを確認します。

索引モデルが issue の本文から書いたものです。