TelephonyBackupAgent: Allow U+0000 in SMS body. am: f56e6480e7 am: 8cc7220b64 am: 6c573bc365

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/providers/TelephonyProvider/+/12685849

Change-Id: Iffe8784986c54f35defeecb3de6bfcb96b3de40c