HealPrediction: Updated anchoring part.

Only myBar and healAbsorbBar should be anchored to self.Health, otherBar is appended to either myBar or self.Health, absorbBar is appended to the last shown bar.
Added .UpdateAnchorOverride(...) hook, just in case someone wants things to be anchored differently.
No need to show bars on enable.
Changed absorb texture to blizz' default.