Prevent DefaultInstaller from working on a non-existent cache file.

Ensures that the cache file exists before attempting an install using
it. This fixes a crash that can occur if the file is not yet available,
likely due to an I/O-related race condition.

src/main/kotlin/com/looker/droidify/installer/DefaultInstaller.kt

@@ -51,6 +51,7 @@ class DefaultInstaller(context: Context) : BaseInstaller(context) {
 
         val session = sessionInstaller.openSession(id)
 
+        if (cacheFile.exists()) {
             session.use { activeSession ->
                 activeSession.openWrite("package", 0, cacheFile.length()).use { packageStream ->
                     cacheFile.inputStream().use { fileStream ->
@@ -62,6 +63,7 @@ class DefaultInstaller(context: Context) : BaseInstaller(context) {
 
                 session.commit(pendingIntent.intentSender)
             }
+        }
         cacheFile.delete()
     }