ExternalCameraHAL: Add ANDROID_SENSOR_READOUT_TIMESTAMP key.

As ExternalCameraHAL does not support sensor readout timestamp,
it should set ANDROID_SENSOR_READOUT_TIMESTAMP to
ANDROID_SENSOR_READOUT_TIMESTAMP_NOT_SUPPORTED in CameraDevice
characteristics.

This CL adds the CameraMetadata entry to ExternalCameraDevice.

Bug: 309543399
Test: ReadoutTimestampTest#testReadoutTimestamp passes for
      ExternalCameraHAL
Change-Id: I12203f0832416b4ca325380e671675ddc1d788d4