Fix for bug 7234184 F/TextLayoutCache: Failed to put an entry...

This bug was triggered by user code concurrently mutating the character
array while calling into a drawText method in another thread. When the
value of the array changed, it caused inconsistent state, leading to
assert failures.

This is arguably bad behavior by the user code, but it shouldn't cause a
native crash. The fix is to do a defensive copy of the text into the
key, so the value is guaranteed to remain constant throughout the text
layout process. The change is mostly deletion of code, because there was
an optimization to try to avoid such a copy. That optimization was not
actually effective, however, because the indexOfKey() operation in the
KeyedVector underlying the TextLayoutCache did the copy anyway. Thus,
even though this change looks like it's introducing a copy where there
wasn't one before, the actual performance impact should be nil.

Note that the ability to handle a mutating argument is now part of the
contract for TextLayoutEngine::getValue(), and is now documented. That
contract may change, as the result of future optimization. Also, care
was taken to only use the value after the copy.

Other performance issues with TextLayoutCache are tracked in bug
7271109.

Change-Id: I9c90e8e4d501f3f37e2f22a7851f032808d46fbe