Keep boot-time logs when the logcat service restarts after a reboot (#1074)

`AndroidDevice.handle_reboot()` stops all services before the reboot and
starts them again once `sys.boot_completed` is set. `Logcat.start()` did
not distinguish that restart from the first start of a run: it cleared the
device buffer (`Config.clear_log` defaults to True) and started
`adb logcat -T 1`, which only follows from the most recent line. Everything
logged between kernel boot and boot completion was therefore dropped from
`adb_logcat_file_path`, and `get_lines()` / `wait_for()` / `listen()` could
not see boot-time events such as the MediaProvider `MEDIA_MOUNTED` scan,
even though they were still available via a raw `adb logcat -d`.

`start()` now reads `/proc/sys/kernel/random/boot_id` and, when it differs
from the id seen by the previous start, treats the call as a restart after a
reboot: the buffer (which only holds this boot's logs) is not cleared and
the new `adb logcat` process dumps the whole buffer before following. A
stop/start on the same boot, or when the boot id cannot be read, keeps the
existing clear-and-follow behavior. `pause()`/`resume()` are unchanged.