Fix code accounting bugs, track external app data.

When counting code size, don't include APKs baked into the system.

Settings already accounts external storage used by apps, so they
need a way to exclude that from the total space used by external
storage; give them new getAppSize() API.

Refine docs to explain that emulated storage might be included in
measured statistics.

Resolve symlinks as part of matching getUuidForPath().

Test: cts-tradefed run commandAndExit cts-dev -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.StorageHostTest
Bug: 35812899, 35844919, 37193650
Change-Id: Iec3ce8b336b71dc98a7d25fdd30fa78e9ee826dc