Use transition-only alpha property for fading transitions

The original bug is fixed already, but showed up some problems in
the underlying fade-transition implementation. This fix addresses
those and other issues. The biggest part of the change should help
transition robustness in general, as it removes the dependency on the
public 'alpha' property of views and uses, instead, a new hidden property
on views called 'transitionAlpha'. This is a value which is normally
opaque (1), but which can be used by transitions (only) to animate the
translucency of views without disturbing the actual 'alpha' value which
might be manipulated outside of transitions. This should make transitions
much more robust in general.

In implementing and testing this overall fix, I noticed a couple of things
about transitions that were simply wrong (such as starting fades from the
wrong start value, and incorrectly avoiding transitions on some views
that didn't happen to have ids), and those are fixed in this CL as well.

Issue #10726905 ActionBar weirdness in People app
Issue #10727937 Menu items in gallery appear in faded color after selecting an image/album by long press

Change-Id: If1618446db10c1bfcff4761449241de4f559afc1

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