HwcClient: Fix plane alpha

Plane alpha was being sent as a float but read as unsigned, which, when
converted to a float was causing things that were supposed to be
translucent to be opaque.

Bug: 33739111
Bug: 33737738
Test: Scrim behind modal dialogs is translucent and there is no more
      glitch when rotating apps

Change-Id: I6896bdbb9928bd1aa7265b3ef25a7a96d54ee227