Cast: Fix thread safety and memory leak in CastMediaRouteProvider

[koczadly]

Problem

CastMediaRouteProvider has no thread synchronisation for its shared mutable state, which can be accessed across numerous threads (NSD callbacks + main thread).
This has been causing ConcurrentModificationException crashes, which seems to be consistently reproducible whenever opening the Google Home app.

Stacktrace

java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1603)
	at java.util.HashMap$ValueIterator.next(HashMap.java:1631)
	at org.microg.gms.cast.CastMediaRouteProvider.publishRoutes(CastMediaRouteProvider.java:329)
	at org.microg.gms.cast.CastMediaRouteProvider.access$500(CastMediaRouteProvider.java:54)
	at org.microg.gms.cast.CastMediaRouteProvider$2.run(CastMediaRouteProvider.java:322)
	at android.os.Handler.handleCallback(Handler.java:995)
	at android.os.Handler.dispatchMessage(Handler.java:103)
	at android.os.Looper.loopOnce(Looper.java:265)
	at android.os.Looper.loop(Looper.java:358)
	at android.app.ActivityThread.main(ActivityThread.java:10049)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:616)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1115)

The inner customCategories collection is also appended to for every discovery request, causing it to grow indefinitely.

Changes

• Add synchronisation by introducing a shared lock object to guard shared state
• Fix unbounded category growth by switching to a Set and replacing contents on each discovery request
• Minor code cleanup

Testing

Just to note, I have the ReVanced version installed on my device as I do not have root-level access. I understand their fork has a few minor differences, though this specific implementation appears to be completely unchanged.

I have deployed these changes onto my personal device and it has been working as expected.

Issues

Fixes #2449