fix opening file explorer apps

[Catfriend1]

Purpose
Fix issue #838
Most file explorers do not implement the "resource/folder" mime type. If the user has such a file explorer installed, e.g. Root Explorer, OI file manager, a second try to find a file manager app that can handle opening the folder is attempted by using the chooserIntent. It will popup a menu like "choose which app you want to use to explore the folder".

Testing
Verified working with root explorer and OI file manager on Android 7.1.2 @lg-h815

[AudriusButkevicius]

This doesn't explain why/how this fixes it.

[Catfriend1]

@AudriusButkevicius I put a brief summary of the problem from the linked issue I intended to fix with this PR on top.

[AudriusButkevicius]

You said that you've implemented a fallback mechanism, but I don't understand why using application/* mime type suddenly fixes this.

[Catfriend1]

@AudriusButkevicius I checked using the debug build (https://build.syncthing.net/repository/download/SyncthingAndroid_Build/19963:id/debug/app-debug.apk). The code detects in resolveActivity that the result activitiy is null (no file manager app found installed that is compatible to folder/resource mime type). Before the change, the code showed a toast error and stopped. Now, it retries with the application/* mime type and the chooser Intent. That can be interpreted as an alternative way with a broader-matching filter. Now it finds a bunch of apps compatible to open (Root Explorer, QuickPic, OI, ...) and shows a selection UI to the user (this is done by ChooserIntent which is from android itself).

[AudriusButkevicius]

I guess this doesn't make it any worse.