Skip restore of read-only directories

Currently trying to restore a read-only directory results in only the
directory being restored but not its contents. This causes problems for
some apps (see the associated bug). Skip restore of read-only directories
while we evaluate the priority of proper suport for this use case.

Bug: 194894879
Test: Manual:
        1. Populate a read-only directory in an app's folder
	(/data/user/0/<package-name>)
	2. Run backup for the app
	3. Clear app data (adb shell pm clear <package-name>
	4. Run restore for the app
	5. Inspect app folder to ensure all writable content is
	restored.
Merged-In: I644a5784fb42352ba28ac944d3aa3564b0215a78
Change-Id: I644a5784fb42352ba28ac944d3aa3564b0215a78
(cherry picked from commit 62c0cec8)