Add and use art::GetAndroidRuntimeBinDir.

This method returns the location of the directory containing the
Android Runtime binaries, which is expected to be the Android Runtime
APEX's bin directory on target and the Android Root's bin directory on
host (at least for now).

Also remove art::CommonArtTestImpl::GetTestAndroidRoot as it is no
longer used.

Test: ART gtests on host and target
Bug: 121117762
Change-Id: I4593bbd4d675ca9d3f7b708482aac82c1892a9ad
diff --git a/runtime/runtime.cc b/runtime/runtime.cc
index dd7c219..ee7e264 100644
--- a/runtime/runtime.cc
+++ b/runtime/runtime.cc
@@ -765,8 +765,10 @@
   if (!compiler_executable_.empty()) {
     return compiler_executable_;
   }
-  std::string compiler_executable(GetAndroidRoot());
-  compiler_executable += (kIsDebugBuild ? "/bin/dex2oatd" : "/bin/dex2oat");
+  std::string compiler_executable = GetAndroidRuntimeBinDir() + "/dex2oat";
+  if (kIsDebugBuild) {
+    compiler_executable += 'd';
+  }
   return compiler_executable;
 }