Fix the abnormal issue of adding calendar to widget

Add calendar to widget it does not show any content, when
first open calendar in widget there is a crash. Fixed by
adding FLAG_IMMUTABLE.

Bug: 398971782

Signed-off-by: Richard Liu <xuegang.liu@nxp.com>
Change-Id: I23e6b74ea74f48c07e02ce15cf8e34c6e84e8d1d