fix(android): merge sideloaded subtitle tracks via MergingMediaSource; guard PiP activity lookup

[WithakErik]

Problem

External subtitle/caption files passed via textTracks never appeared on Android. Two related bugs:

Bug 1 — Subtitles silently ignored
ProgressiveMediaSource.Factory (and the HLS/DASH variants) ignore MediaItem.SubtitleConfiguration entries entirely. Subtitles set via setSubtitleConfigurations are attached to the MediaItem but never loaded into MappedTrackInfo, so setSelectedTextTrack has nothing to select.

Reported in #4792 and #4614.

Bug 2 — PiP crash after subtitle change
findActivity() threw IllegalStateException when the ComponentActivity wasn't reachable through the ContextWrapper chain. This surface became more likely after the subtitle fix changes ExoPlayer's internal session/lifecycle state via MergingMediaSource.

Fix

Subtitle fix (ReactExoplayerView.java)

After the primary MediaSource is built, check for SubtitleConfiguration entries on the MediaItem. For each one, create a dedicated ProgressiveMediaSource backed by a SubtitleExtractor, then merge all sources with MergingMediaSource. This makes sideloaded tracks appear in MappedTrackInfo and respond to setSelectedTextTrack exactly like embedded tracks.

if (mediaItem.localConfiguration != null && !mediaItem.localConfiguration.subtitleConfigurations.isEmpty()) {
    // build one ProgressiveMediaSource per subtitle, merge with video source
    mediaSource = new MergingMediaSource(mergedSources);
}

PiP fix (PictureInPictureUtil.kt)

• findActivity() now returns ComponentActivity? instead of throwing, with a ThemedReactContext.getCurrentActivity() fallback
• All call sites updated to handle null with early returns
• Replaces deprecated context.currentActivity field with context.getCurrentActivity()

Testing

Verified on a physical Android device (arm64-v8a) with external .vtt subtitle files. Subtitles now load and are selectable. PiP entry/exit no longer crashes after subtitles are active.

Closes #4792
Related: #4614

[WithakErik]

Updated with three hardening improvements (based on additional code review):

• Strip subtitle configs from base MediaItem before passing to mediaSourceFactory — prevents any factory variant from also creating legacy subtitle sources alongside our MergingMediaSource, which would trigger the Media3 1.8 Legacy decoding is disabled crash
• Guard unsupported formats via subtitleParserFactory.supportsFormat() before calling create() — unknown MIME types previously threw IllegalArgumentException at source creation time, killing playback; now we log a warning and skip the track, matching DefaultMediaSourceFactory's behavior
• Preserve selectionFlags and roleFlags on the rebuilt Format so default/forced-caption auto-selection continues to work correctly