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

DocumentsProvider: handle mount IOException

CephMount: add missing IOException throw to mount()

add option to bypass file permission

build: update toolchain

support create