Move initialization from onCreate to attachBaseContext

Previously we initialize WindowManager and associate DisplayArea in onCreate,
but apps may use WindowManager before WindowProviderService#onCreate.
The code may be like:
```
public void onCreate() {
    WindowManager wm = getSystemService(Windowmanager.class);
    wm.getCurrentWindowMetrics().getBounds(); // <--- crash here
    super.onCreate()
    ...
}
```
This CL move the initialization step to attachBaseContext to prevent
the crash.

Test: atest WindowContextTests
Test: run gsi_avd/boot_health/vendor_img_rvc on ABTD
Bug: 191635763
Change-Id: Id8bad73a749813a90b307ac2b4d020f740631ea3