Fix dist(s)-with-tag and add tests to prevent regression.

While AndroidMkEntries' DistFiles were populated from java_library
properly, there was a bug in the conditionals that dropped the tagged
DistFiles.

Fixes b/152834186
Fixes b/161339864

Test: soong tests, m sdk dist (creates out/dist/test.jar with
aosp/1363578)

Signed-off-by: Jingwen Chen <jingwen@google.com>
Change-Id: Id895282256fd367b2f46dcb7a643465fa66c9456
diff --git a/android/androidmk.go b/android/androidmk.go
index dfc68c4..94b4b81 100644
--- a/android/androidmk.go
+++ b/android/androidmk.go
@@ -165,7 +165,7 @@
 	var ret []string
 
 	availableTaggedDists := TaggedDistFiles{}
-	if a.DistFiles != nil && len(a.DistFiles[""]) > 0 {
+	if a.DistFiles != nil {
 		availableTaggedDists = a.DistFiles
 	} else if a.OutputFile.Valid() {
 		availableTaggedDists = MakeDefaultDistFiles(a.OutputFile.Path())