graphics: use allocator HAL support library in default impl

Rename Gralloc0Allocator to Gralloc0Hal and make it inherit from
AllocatorHal.  Do the same to Gralloc1Allocator.  Add GrallocLoader
to load either of Gralloc[01]Hal and create a IAllocator instance.

Test: boots and VTS
Change-Id: I09ae680c0086ca9e73e412a34d7cd2f3665d3bc2
diff --git a/graphics/allocator/2.0/default/Android.bp b/graphics/allocator/2.0/default/Android.bp
index f100d96..ed2ecbb 100644
--- a/graphics/allocator/2.0/default/Android.bp
+++ b/graphics/allocator/2.0/default/Android.bp
@@ -3,10 +3,13 @@
     defaults: ["hidl_defaults"],
     vendor: true,
     relative_install_path: "hw",
-    srcs: ["Gralloc.cpp"],
+    srcs: ["passthrough.cpp"],
     static_libs: [
         "android.hardware.graphics.allocator@2.0-passthrough",
     ],
+    header_libs: [
+        "android.hardware.graphics.allocator@2.0-hal",
+    ],
     shared_libs: [
         "android.hardware.graphics.allocator@2.0",
         "libbase",