Add test for stat() on app private dir.

It should not be possible to call stat() on another app's private data
directory.

Modified the test, and its helper, to target SDK 29; the protection
mechanism changes significantly in Android 11 for apps targeting API
level 30+. CTS tests for the new mechanism are in
AppDataIsolationTests.

Fixed various misleading comments.

Bug: 161356067
Test: atest android.appsecurity.cts.AppSecurityTests#testAppFailAccessPrivateData_full
Test: atest android.appsecurity.cts.AppSecurityTests#testAppFailAccessPrivateData_instant
Change-Id: I4f55003cc9b8d2c411c24313c468623fef7f4317
(cherry picked from commit 4de40f50f14ed7d492a5e5d52b8cfcaedf925e9d)