Report battery level to other nodes in mesh (and apps).

Also today I'll turn back on the the standard "has a battery" BLE
device profile, so the locally connected radio should show up in the
phone's bluetooth popup of battery levels (similar to how a BLE
headphone battery level appears)

cc @lgoix - using code from @professr

```
meshtastic --info
Connected to radio...
my_node_num: 2883444536
has_gps: true
num_channels: 13
region: "unset"
hw_model: "tbeam"
firmware_version: "unset"
packet_id_bits: 32
current_packet_id: 91018534
node_num_bits: 32
message_timeout_msec: 300000
min_app_version: 172

preferences {
  position_broadcast_secs: 900
  send_owner_interval: 4
  wait_bluetooth_secs: 120
  screen_on_secs: 300
  phone_timeout_secs: 900
  phone_sds_timeout_sec: 7200
  mesh_sds_timeout_secs: 7200
  sds_secs: 31536000
  ls_secs: 3600
}
channel_settings {
  tx_power: 23
  modem_config: Bw125Cr48Sf4096
  psk: "\324\361\273: )\007Y\360\274\377\253\317Ni\277"
  name: "Default"
}

Nodes in mesh:
{'num': 2883444536, 'user': {'id': '!2462abdddf38', 'longName': 'Unknown df38', 'shortName': '?38', 'macaddr': 'JGKr3d84'}, 'position': {'altitude': 96, 'batteryLevel': 100, 'latitudeI': 375210641, 'longitudeI': -1223090398, 'time': 1592843564, 'latitude': 37.5210641, 'longitude': -122.3090398}}
```

docs/software/TODO.md

@@ -2,6 +2,7 @@
 
 You probably don't care about this section - skip to the next one.
 
+- report battery level to android app
 - @feh123 Sony Xperia Z1 C6903 running Android 5.1.1
 - don't stop advertising BLE
 - first message sent is still doubled for some people