Deprecate Content::getNativeData, add TextContent::getText

Deprecate Content::getNativeData, add TextContent::getText

getNativeData() is under-specified - callers can do nothing with the
value returned by getNativeData without knowing the concrete Content
class. And if they know the concrete class, they can and should use
a specialized getter instead, anyway.

Basically, getNativeData is overly generic, an example of polymorphism
done poorly. Let's fix it now.

Bug: T155582
Change-Id: Id2c61dcd38ab30416a25746e3680edb8791ae8e8