We provide a sparse file directly as a (writable) partition image and
format a filesystem on the partition inside the VM.
To format it only when it's not formatted, the option `formattable` is
added to the fstab entry for the userdata partition.
Bug: 185767624
Test: atest MicrodroidHostTestCases
Change-Id: Ia938fd60071e64170b8280d19c17c4413c769337