Support setting target_sdk_version separately from sdk_version

Before this change, if targetSdkVersion wasn't set in the
AndroidManifest.xml, we'd set it to the sdk_version from the Android.bp.

But there are cases where you want to compile against a later SDK, but
target an earlier one (especially if you depend on libraries that need
to be compiled against more recent SDKs, like androidx).

Test: build APK with different target_sdk_version.
Change-Id: Iaed36b522955a374a049ef331158cc8fc5798ad2
diff --git a/java/aar.go b/java/aar.go
index 3ab07cc..a49aef0 100644
--- a/java/aar.go
+++ b/java/aar.go
@@ -413,6 +413,10 @@
 	return a.sdkVersion()
 }
 
+func (a *AARImport) targetSdkVersion() string {
+	return a.sdkVersion()
+}
+
 var _ AndroidLibraryDependency = (*AARImport)(nil)
 
 func (a *AARImport) ExportPackage() android.Path {