mkdir .incomplete in backing storage to fix CTS/GTS failures

Linux kernel included a commit 17efe505a4c3df88d3e13051f421d27d7178d47f
that creates .incomplete folder on mounting incfs. However .incomplete
folder is only accessible for root with 0700 permission. So system_server
has no permission to access it.
By creating .incomplete folder with 0777 permission in system_server,
incfs won't have to create incomplete folder implicitly.

Bug: 176400491
Test: CtsIncrementalInstallHostTestCases
          CtsContentTestCases android.content.pm.cts.PackageManagerShellCommandIncrementalTest
          CtsStatsdHostTestCases android.cts.statsd.atom.UidAtomTests#testPackageInstallerV2MetricsReported
          GtsIncrementalInstallProxyHostTestCases

Change-Id: Icb7babe3b482a089019f981fe149de7d619ce87e