ui/app: (android) block until DataDir is ready

The app data dir is not set until after Go's init functions have
run, which means that DataDir is inherently racy. Avoid that race
by blocking in DataDir until it is set from Java.
In other words, trade a race condition with a deadlock.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/app/datadir_android.go

@@ -5,13 +5,23 @@
 package app
 
 import "C"
+import "sync"
 
-var dataDir func() (string, error)
+var (
+	dataDirOnce sync.Once
+	dataDirChan = make(chan string, 1)
+	dataPath    string
+)
+
+func dataDir() (string, error) {
+	dataDirOnce.Do(func() {
+		dataPath = <-dataDirChan
+	})
+	return dataPath, nil
+}
 
 //export setDataDir
 func setDataDir(cdir *C.char, len C.int) {
 	dir := C.GoStringN(cdir, len)
-	dataDir = func() (string, error) {
-		return dir, nil
-	}
+	dataDirChan <- dir
 }