Change static singletons to cached singletons with weak references to save memory when the objects are not needed anymore

WebDAV mounts: add empty view; add navigation drawer entry for Managed DAVx5 (closes bitfireAT/davx5#23)

WebDAV: accept compliance class 2 or 3 (closes bitfireAT/davx5#21); fix division by zero in progress

- WebDAV service detection: accept compliance class 2 or 3, because they both imply class 1 (there are servers which don't show class 1 although they MUST)
- WebDAV random access: fix division by zero when file size reported by HEAD is zero

Update cert4android and vcard4android (only README changes); Android10Resolver: use empty JVM object instead of UUID as handle

With the recent move to GitHub, the AboutInfo override for dav4jvm was
broken as it still used com_gitlab_bitfireAT__dav4jvm whereas it is now:
```
$ ./gradlew findLibraries | grep dav4jvm
bitfireAT/dav4jvm (f1d9354a41) -> com_github_bitfireAT__dav4jvm
Manually requested license: mpl_2_0
```

Since the autogenerated dav4jvm AboutInfo looks fine (see attached
screenshot), remove the override.

Follow-up for commit 331c1029

(trim deletes the files in modification time order, so if two files have the same
modification time, the order is not well-defined)

- Android 8 to 10: use all DNS servers; prioritize resolvers of active connections
  (before: use only DNS servers of "active" connection, which may be a VPN connection without servers)
- Android 10+: use new DnsResolver API so that we don't need to know DNS servers anymore (also supports DoT etc.)

When displaying the notification, we use serviceId as tag, so use
serviceId as well when cancelling it.