android_ab: don't ignore ab_control_store return code

ab_control_store() can return an error if writing to disk fails.
In this case, we have to pass the error code to the caller.

Signed-off-by: Alexey Romanov <avromanov@salutedevices.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>