Discourage use of "_data" column.

Moving forward, all client file access really needs to be going
through explicit APIs like openFileDescriptor(), since that allows
the provider to better protect its underlying files.

This change also changes several classes to use the AutoClosable
pattern, which enables try-with-resources usage.  Older release()
methods are deprecated in favor of close().

Uniformly apply CloseGuard across several classes, using
AtomicBoolean to avoid double-freeing, and fix several resource
leaks and bugs related to MediaScanner allocation.  Switch
MediaScanner and friends to use public API instead of raw AIDL calls.

Bug: 22958127
Change-Id: Id722379f72c9e4b80d8b72550d7ce90e5e2bc786