minadbd: Return battery level via getprop.

`adb rescue getprop rescue.battery_level` returns the current battery
level.

Bug: 134560109
Test: Build and boot into rescue mode.
Test: `adb rescue getprop ro.build.fingerprint`
Test: `adb rescue getprop rescue.battery_level`
Test: `adb rescue getprop` to dump all the properties.
Test: Run recovery_unit_test.
Change-Id: I78a9e8ab9783ffc8532cb93e6a64fb2157c19bd5