Initial prebuilt support

Support prebuilt shared libraries as an initial proof-of-concept of
prebuilts.  Future changes will support binaries and static libraries,
and the ability to select which to use based on something besides
blueprint properties.

Test: TestPrebuilts run during m -j
Change-Id: I6e84da667e9005ae11844bad01d25cbe4ced1ce3
diff --git a/Android.bp b/Android.bp
index 0ba669c..8821098 100644
--- a/Android.bp
+++ b/Android.bp
@@ -72,6 +72,7 @@
         "android/onceper.go",
         "android/package_ctx.go",
         "android/paths.go",
+        "android/prebuilt.go",
         "android/register.go",
         "android/util.go",
         "android/variable.go",
@@ -81,6 +82,7 @@
     ],
     testSrcs: [
         "android/paths_test.go",
+        "android/prebuilt_test.go",
     ],
 }
 
@@ -126,6 +128,7 @@
         "cc/check.go",
         "cc/gen.go",
         "cc/makevars.go",
+        "cc/prebuilt.go",
         "cc/relocation_packer.go",
         "cc/sanitize.go",
         "cc/stl.go",