Add tombstone_handler for crashes in Microdroid

This changes the crash export mechanism in Microdroid. For this, we
create module tombstone_handler which exports methods very similar to
tombstoned.h

For Microdroid (detected using prop: ro.hardware): It calls newly
introduces microdroid specific methods to connect/notify completion of
crash.

Individual methods:
connect -> For Android, it would connect to
tombstoned which would send it the fd corresponding to newly created
file on /data/tombstone_ . For Microdroid, we connect to tombstone
server on host via vsock & populate these sockets as the output fd.
crash_dump, in the later case, would directly write on the socket(s).

notify_completion: For Microdroid, it would simply shutdown the vsock
connections.

Note when OS is not Microdroid: It calls corresponding methods of
tombstoned_client, essentially serving as a proxy.

Detailed design: go/vm-tombstone
Test: atest MicrodroidHostTests#testTombstonesAreGeneratedUponUserspaceCrash
Bug: 243494912

Change-Id: I68537b967f2ee48c1647f0f923aa79e8bcc66942