Add AHB and Pixel- Formats for R8

Add AHARDWAREBUFFER_FORMAT_R8_UNORM and PIXEL_FORMAT_R_8. These will be
used for an alpha 8 buffer representing the screen decorations.

Enum value matches AIDL R_8 in
I6f6b7f22e76609151a413ca6a6dbf55bd91911dd.

Bug: 193170859
Test: make
Change-Id: I4846327b29736ee811672801a683deb287342a8e
diff --git a/libs/ui/PixelFormat.cpp b/libs/ui/PixelFormat.cpp
index d1925cb..799fbc9 100644
--- a/libs/ui/PixelFormat.cpp
+++ b/libs/ui/PixelFormat.cpp
@@ -35,6 +35,8 @@
         case PIXEL_FORMAT_RGBA_5551:
         case PIXEL_FORMAT_RGBA_4444:
             return 2;
+        case PIXEL_FORMAT_R_8:
+            return 1;
     }
     return 0;
 }