Allow making AssetFileDescriptors from SQLite blobs.

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