Enable mute test pattern for external cameras

 Add ANDROID_SENSOR_TEST_PATTERN_MODE_SOLID_COLOR as an available
 test pattern for ExternalCameraDevice (v3.4).

Bug: 182204067
Test: Connect an USB camera.
 Enable camera privacy toggle.
 Apps using the camera should get a black preview
Change-Id: Ie0d4d5b3f555eb9e1b1fc255b612668736daeaa4