Add app's own UID to the default ACL.

Add app's own UID to the default ACL.

On devices without sdcardfs, /Android/data/com.foo and
/Android/obb/com.foo can be written by other processes (eg installers);
in those cases, file ownership may be wrong. To ensure that the original
app always has access to the files contained in this directory, add a
group to the default ACL that matches the UID of the app. Since all apps
have their own UID also as their group ID, this ensures that things keep
working correctly.

Bug: 157530951
Test: atest
android.appsecurity.cts.ExternalStorageHostTest#testExternalStorageUnsharedObb
Change-Id: I829a2a7c7b578a8328643f38681e68796adcd6b2

Change-Id: Ibbc333fb395507363830dfcf5dc6f1cfd55f008d