MicrodroidHostTestCases now uses the vm tool to create the VM. It no
longer directly interacts with crosvm and mk_cdisk.

The READMD.md file is rewritten to fully reflect the recent changes, and
also to add description about building an app for microdroid.

Bug: 185891097
Test: atest MicrodroidHostTestCases
Change-Id: I5fdc854390fd362ebead22a4a36af75c30500a81