Add an app size collector.

The app collector gets a list of app sizes for packages on a given
storage volume. This information will be exposed as part of an
expansion of the diskstats dumpsys.

When the collector runs, it sets up a handler on a BackgroundThread
which asks the PackageManager for the package sizes for all apps and
all users. The call for the information is blocked using a
CompletableFuture until the call times out or until we've received
all of the package stats. After the stats are all obtained, the
future completes.

Bug: 32207207
Test: System server instrumentation tests
Change-Id: I3a27dc4410effb12ae33894b561c02a60322f7b0