Fix thread access when creating notifications for media sessions

The sessions may have different application threads for their players,
and the service with its notification provider runs on the main thread.
To ensure everything runs on the correct thread, this change labels
methods where needed and fixes thread access in some places.

Issue: androidx/media#318
PiperOrigin-RevId: 524849598

Author: tonihei

testutils/build.gradle

@@ -20,6 +20,7 @@ dependencies {
     api 'androidx.test.ext:truth:' + androidxTestTruthVersion
     api 'junit:junit:' + junitVersion
     api 'com.google.truth:truth:' + truthVersion
+    api 'com.google.truth.extensions:truth-java8-extension:' + truthVersion
     compileOnly 'org.checkerframework:checker-qual:' + checkerframeworkVersion
     compileOnly 'org.checkerframework:checker-compat-qual:' + checkerframeworkCompatVersion
     compileOnly 'org.jetbrains.kotlin:kotlin-annotations-jvm:' + kotlinAnnotationsVersion