ANDROID: Incremental fs: Fix pseudo-file attributes

Prior change

ANDROID: Incremental fs: stat should return actual used blocks

adds blocks to getattr. Unfortunately the code always looks for the
backing file, and pseudo files don't have backing files, so getattr
fails for pseudo files.

Bug: 186567511
Test: incfs_test passes, can do incremental installs on test device
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: Ia3df87f3683e095d05c822b69747515963c95f1c