Offer to cache ContentResolver-related Bundles.

There are a handful of core system services that collect data from
third-party ContentProviders by spinning them up and then caching the
results locally in memory.  However, if those apps are killed due to
low-memory pressure, they lose that cached data and have to collect
it again from scratch.  It's impossible for those apps to maintain a
correct cache when not running, since they'll miss out on Uri change
notifications.

To work around this, this change introducing a narrowly-scoped
caching mechanism that maps from Uris to Bundles.  The cache is
isolated per-user and per-calling-package, and internally it's
optimized to keep the Uri notification flow as fast as possible.
Each Bundle is invalidated whenever a notification event for a Uri
key is sent, or when the package hosting the provider is changed.

This change also wires up DocumentsUI to use this new mechanism,
which improves cold-start performance from 3300ms to 1800ms.  The
more DocumentsProviders a system has, the more pronounced this
benefit is.  Use BOOT_COMPLETED to build the cache at boot.

Add more permission docs, send a missing extra in DATA_CLEARED
broadcast.

Bug: 18406595
Change-Id: If3eae14bb3c69a8b83a65f530e081efc3b34d4bc