Add @SystemApi annotation to parcel.writeBlob and readBlob.

AppSearch will put text-based document via binder to server side.
Parcel.writeBlob could take care of decide where to put in Android
Shared Memory if data is too large.
Remove {@SystemApi} to make it available for AppSearch.

Bug: 185441119
Test: presubmit
Change-Id: I95218fdca2baa5b4a287f05c9d79273b4a6e622c