Wire up storage stats API to installd.

Now that installd has the implementation details we need, we can wire
up the public APIs to use them.

Shuffle APIs around a bit so that StorageStats can be reused for both
UID and UserHandle results, and rename StorageSummary to
ExternalStorageStats.  Provide getTotalBytes() and getFreeBytes() as
first-class methods so we can answer those questions quickly without
paying the cost of measuring external storage details.

Current costs on a typical device with a test account after flushing
dentry caches:

queryStatsForUid() manual: 6922ms
queryStatsForUid() quota: 525ms

queryStatsForUser() manual: 1686ms
queryStatsForUser() quota: 113ms

queryExternalStatsForUser() manual: 42ms
queryExternalStatsForUser() quota: 2ms

For verification purposes, a new "fw.verify_storage" system property
can be set to compute both manual and quota statistics, and log any
discrepancies.

Test: builds, boots
Bug: 27948817, 32206268
Change-Id: I4ea3df3372a7379aa8cf4c20c44120c8f0702c15