Update AssistStructure.ViewNode#getMaxTextLength() javadoc.

Since S, we limit characters for single line TextView due to the
performance reason(459f35ac). The return value for not set case is
not -1. Update the API javadoc to prevent giving wrong information.

Bug: 159037010
Test: n/a
Change-Id: Ic5aa93083f48fa50e097579e61320cc13897265f