refactor(concurrent): Introduce SequentialJob to manage service setup (#3983)

Signed-off-by: James Rich <2199651+jamesarich@users.noreply.github.com>

app/src/main/java/com/geeksville/mesh/concurrent/SequentialJob.kt

@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) 2025 Meshtastic LLC
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package com.geeksville.mesh.concurrent
+
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Job
+import java.util.concurrent.atomic.AtomicReference
+import javax.inject.Inject
+
+/**
+ * A helper class that manages a single [Job].
+ *
+ * When a new job is launched, the previous one is cancelled. This is useful for ensuring that only one operation of a
+ * certain type is running at a time.
+ */
+class SequentialJob @Inject constructor() {
+    private val job = AtomicReference<Job?>(null)
+
+    /**
+     * Cancels the previous job (if any) and launches a new one in the given [scope].
+     *
+     * The new job uses [handledLaunch] to ensure exceptions are reported.
+     */
+    fun launch(scope: CoroutineScope, block: suspend CoroutineScope.() -> Unit) {
+        cancel()
+        val newJob = scope.handledLaunch(block = block)
+        job.set(newJob)
+
+        newJob.invokeOnCompletion { job.compareAndSet(newJob, null) }
+    }
+
+    /** Cancels the current job. */
+    fun cancel() {
+        job.getAndSet(null)?.cancel()
+    }
+}