Detect wedged ContentProviders, treat as ANR.

All ContentProvider calls are currently blocking, making it hard for
an app to recover when a remote provider is wedged.  This change adds
hidden support to ContentProviderClient to timeout remote calls,
treating them as ANRs.  This behavior is disabled by default.

Update DocumentsUI to use a 20 second timeout whenever interacting
with a storage provider.

Bug: 10993301, 10819461, 10852518
Change-Id: I10fa3c425c6a7225fff9cb7a0a07659028230cd3