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
(cherry picked from commit 0df10c4e41dd74f52ec0e17745ff6628d048e0fa)
diff --git a/graphics/composer/2.1/default/HwcClient.cpp b/graphics/composer/2.1/default/HwcClient.cpp
index 16af94c..5599959 100644
--- a/graphics/composer/2.1/default/HwcClient.cpp
+++ b/graphics/composer/2.1/default/HwcClient.cpp
@@ -934,7 +934,7 @@
         return false;
     }
 
-    auto err = mHal.setLayerPlaneAlpha(mDisplay, mLayer, read());
+    auto err = mHal.setLayerPlaneAlpha(mDisplay, mLayer, readFloat());
     if (err != Error::NONE) {
         mWriter.setError(getCommandLoc(), err);
     }