SDK support improvements.

Move figuring out the path to aidl/aapt/dx/llvm-rs-cc from IAndroidTarget
to SdkParser (in preparation of the build-tools).

Also change android.target to android.compileSdkVersion to make it
less confusing with regards to min/targetSdkVersion and also change it
to take either an int or a string.

Change-Id: I8642cb84f67cf75b49abe905b2a65f4d26668e98
diff --git a/tests/repo/library/build.gradle b/tests/repo/library/build.gradle
index 90ed7b6..84c8d6f 100644
--- a/tests/repo/library/build.gradle
+++ b/tests/repo/library/build.gradle
@@ -19,7 +19,7 @@
 }
 
 android {
-    target 'android-15'
+    compileSdkVersion 15
 }
 
 group = 'com.example.android.multiproject'