Guard startForeground calls with try/catch and add isForeground flag

[Ludevv]

Problem

Google Play Console flags our production releases with a "Restricted foreground service types" warning, pointing at com.brentvatne.exoplayer.VideoPlaybackService.onStartCommand. Apps targeting Android 15+ cannot start mediaPlayback foreground services from BOOT_COMPLETED receivers, and Play's static analysis flags the unconditional startForeground() call as a potential crash path

Solution

• Port the foreground-start guard from the v7 player service: the onStartCommand change is a copy of v7's isForeground flag + try/catch around the placeholder notification
• Apply the same pattern to registerPlayer's startForeground() call - this call site only exists in the legacy service (v7 uses setMediaNotificationProvider instead)

cc: @sharnik @szymonkoper @mobily