Add toString() to prevent IllegalAccessException.

If RILJ tries to print log after handling getSimPhonebookCapacityResponse,
IllegalAccessException occurs because AdnCapacity doesn't have toString.

Bug: 235530257
Test: Manual

Signed-off-by: Yunjin Kim <yunzhen.kim@samsung.com>
Change-Id: I496d9aba5369f13be32dcdb89abe508fc07a5603