ANDROID: Incremental fs: set the correct access to mapped files

Backing file needs to have write permissions for all users
even though the mounted view doesn't - otherwise incfs can't
change the internal file data.

Bug: 180535478
Test: adb install <apk>
Signed-off-by: Yurii Zubrytskyi <zyy@google.com>
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: Ie25ff62b5b78a2f1f52de68d9b39c3a9825af82b