This change adds a new class SQLiteContentHelper, which contains
a static method for creating an AssetFileDescriptor from
an SQLite query that returns a blob.
Internally, this uses a file descriptor for a MemoryFile.
The implementation is temporary. Ideally, the data should be copied
directly from SQLite to the MemoryFile ashmem region, using
sqlite3_blob_read().
This is part of the implementation of http://b/issue?id=1871731
c55e08e3fc