Clarify AnimationSet docs and behavior

Two problems fixed here:
- The docs for AnimationSet were too vague and incorrect: some properties of
Animation (such as duration) are pushed from the AnimationSet down to its children, as
the current docs say. Some other properties (such as repeatCount) are ignored. Other
properties (such as startOffset) apply to the Set itself, but not to the children.
Fix: clarify this behavior for each of the properties.
- The behavior for XML resources was just busted. We would set the various properties
(e.g., duration) and then forget that we did so, since we reset the flags that marked
their existence after we loaded the resource. In fact, the duration property was
always being reset to 0, regardless of what it was set to in the xml resource.
Fix: Make it work they way it always should have: respect the values read from the XML
resource and make them behave the same way they do when set at runtime.

Change-Id: I07d68876d2259105dc5a359501d5c656ecfaa8e5