soong: Use the CpExecutable rule for copying libraries

If you compile a shared library with 'cc_library_shared' it will
be executable. If you build one with 'cc_prebuilt_library_shared'
it will not. Use CpExecutable so it's consistent.

Test: Observe that all cc_prebuilt_library_shared modules
      are executable.
Change-Id: I3d7edfc920760aa2f73e4527d4cfab1baa1523fc
diff --git a/cc/prebuilt.go b/cc/prebuilt.go
index a4ca590..e2cd3ad 100644
--- a/cc/prebuilt.go
+++ b/cc/prebuilt.go
@@ -178,7 +178,7 @@
 				implicits = append(implicits, importLibOutputFile)
 
 				ctx.Build(pctx, android.BuildParams{
-					Rule:        android.Cp,
+					Rule:        android.CpExecutable,
 					Description: "prebuilt import library",
 					Input:       importLibSrc,
 					Output:      importLibOutputFile,
@@ -189,7 +189,7 @@
 			}
 
 			ctx.Build(pctx, android.BuildParams{
-				Rule:        android.Cp,
+				Rule:        android.CpExecutable,
 				Description: "prebuilt shared library",
 				Implicits:   implicits,
 				Input:       in,