Allow creating AssetFileDescriptors for MemoryFiles.

This allows content providers to use in-memory data to implement
ContentProvider.openAssetFile(), instead of just normal files
and sockets as before.

To test cross-process use of AssetFileDescriptors for MemoryFiles,
a test content provider and a client for it are added to
AndroidTests.

Fixes http://b/issue?id=1871731