Make WorkLockActivity theme translucent

* Instead, set the background in the auth_biometric_background view
* This fixes a bug where there is an infinite loop to start the confirm device credential intent

Bug: 234536059
Test: Manual testing with TestDPC

Manual testing steps
* Setup device with TestDPC
* Set a work lock
* Open TestDPC in the work profile
* Select "lock now"
* Verify there is no infinite loop

Change-Id: I866e3d48a66e562050ad89ebc7f2ae0ac0455737

packages/SystemUI/AndroidManifest.xml

@@ -723,7 +723,7 @@
                   android:excludeFromRecents="true"
                   android:stateNotNeeded="true"
                   android:resumeWhilePausing="true"
-                  android:theme="@style/Theme.AppCompat.DayNight.NoActionBar">
+                  android:theme="@android:style/Theme.Translucent.NoTitleBar.Fullscreen">
             <intent-filter>
                 <action android:name="android.app.action.CONFIRM_DEVICE_CREDENTIAL_WITH_USER" />
                 <category android:name="android.intent.category.DEFAULT" />

packages/SystemUI/res/layout/auth_biometric_background.xml

@@ -16,6 +16,8 @@
   -->
 <LinearLayout
     xmlns:android="http://schemas.android.com/apk/res/android"
+    android:theme="@style/Theme.AppCompat.DayNight"
+    android:background="?android:colorBackground"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:gravity="center"