ANDROID: Incremental fs: Add setattr call

As was, chmod would change the cached inode's mode, which would
persist until the inode was uncached.

Fix to change mode of backing file, but make sure mount files
are read only, backing files are always writeable.

Test: App no longer fails with incfs errors
Bug: 154972299
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: I40517331f24329484387c6b880f1517f887b29f6