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/android_manifest.go b/java/android_manifest.go
index 168a22d..36f24ff 100644
--- a/java/android_manifest.go
+++ b/java/android_manifest.go
@@ -58,7 +58,7 @@
 		Output: fixedManifest,
 		Args: map[string]string{
 			"minSdkVersion":    sdkVersionOrDefault(ctx, sdkContext.minSdkVersion()),
-			"targetSdkVersion": sdkVersionOrDefault(ctx, sdkContext.sdkVersion()),
+			"targetSdkVersion": sdkVersionOrDefault(ctx, sdkContext.targetSdkVersion()),
 			"args":             strings.Join(args, " "),
 		},
 	})