minadbd: Update README.md.

minadbd is now mostly built from libadbd. The fuse features are unique to

minadbd, and services.c has been modified as follows:

  - all services removed

  - all host mode support removed

  - `sideload_service()` added; this is the only service supported. It

    receives a single blob of data, writes it to a fixed filename, and

    makes the process exit.

minadbd

=======

`minadbd` is analogous to the regular `adbd`, but providing the minimal services to support

recovery-specific use cases. Generally speaking, `adbd` = `libadbd` + `libadbd_services`, whereas

`minadbd` = `libadbd` + `libminadbd_services`.

Although both modules may be installed into the recovery image, only one of them, or none, can be

active at any given time.

- The start / stop of `adbd` is managed via system property `sys.usb.config`, when setting to `adb`

  or `none` respectively. Upon starting recovery mode, `adbd` is started in debuggable builds by

  default; otherwise `adbd` will stay off at all times in user builds. See the triggers in

  `bootable/recovery/etc/init.rc`.

- `minadbd` is started by `recovery` as needed.

  - When requested to start `minadbd`, `recovery` stops `adbd` first, if it's running; it then forks

    and execs `minadbd` in a separate process.

  - `minadbd` talks to host-side `adb` server to get user requests.

    - `minadbd` handles some requests directly, e.g. querying device properties for rescue service.

    - `minadbd` communicates with `recovery` to fulfill requests regarding package installation. See

      the comments in `bootable/recovery/install/adb_install.cpp` for the IPC protocol between

      `recovery` and `minadbd`.

  - Upon exiting `minadbd`, `recovery` restarts `adbd` if it was previously running.