add editorconfig

CephFSDocumentsProvider: root: improve naming

make title shorter, and move cephfs string to summary, which makes it
friendlier to stock DocumentsUI with phone screen

jni: update prebuilts from ceph v16.2.7 rebase, un-lfs

newer ndk r23.b toolchain seems to produce a lot smaller binaries, and
we can now get rid of lfs. old binraries are branched at
old-prebuilts-lfs branch on github.

build: require sdk runtime minimum 24

this restriction actually already exists for a long time, from libceph*
libraries. ceph needs:

ns_* from arpa/nameser.h, __INTRODUCED_IN_64(22)
{get,free}ifaddrs from ifaddrs.h,  __INTRODUCED_IN(24)

CephFSDocumentsProvider: lookup mime-type on symlinks

build: use maven central instead of jcenter

make mount timeout configurable

DocumentsProvider: toast and unset global handle on mount errors

update build toolset

update prebuilt ceph libs to pacific

DocumentsProvider: setup ceph only when needed

DocumentsProvider: set ceph mount timeout to 20

DocumentsUI on Pixels seems to have a timeout of 20.
Default timeout of 300 is too long, let's make it 20 for now.
To be configurable in future.

DocumentsProvider: unmount before new mount

build: update gradle plugin

DocumentsProvider: isChildDocument support

This add support for ACTION_OPEN_DOCUMENT_TREE

DocumentsProvider: toast on IOException

DocumentsProvider: don't hardcode log tag everywhere

ProxyFileDescriptorCallback: handle IOException

DocumentsProvider: set up ceph on create

DocumentsProvider: deduplicate ceph operation error handling