Avoid readCString crashes.

Reading nullptr here would cause a crash, killing storaged. Use readString8/writeString8 instead. This may treat malformed UTF-8
Strings differently.

Please go/aidl-it (convert to AIDL-generated parcelable).

Bug: 301519740
Change-Id: I52aff7f3e0e300cce6334b45398c6671a6a0430e
Test: TH