Fix HLS load position reset when upstream chunks are discarded

When resuming HLS low-latency playback after a temporary playlist load failure, if a full segment is available on the server and its `#EXT-X-PART` tags are removed from the playlist, `getChunkPublicationState` marks the buffered partial chunks as `CHUNK_PUBLICATION_STATE_REMOVED`.

In `discardUpstream`, purging all partial chunks when `mediaChunks.isEmpty()` resets `pendingResetPositionUs` directly to `lastSeekPositionUs` (e.g. 60s earlier). The `HlsChunkSource` would refill the buffer from that old position, triggering an unexpected audio track timestamp discontinuity exception in `DefaultAudioSink`.

By assigning `pendingResetPositionUs = max(lastSeekPositionUs, firstRemovedChunk.startTimeUs)`, we ensure ExoPlayer reloads from the start time of the removed upstream chunk while protecting against post-seek keyframe snapping (`max`).

#cherrypick

Issue: androidx/media#3311
PiperOrigin-RevId: 947616837
(cherry picked from commit b842f19baeaa2f6add0b4b559cacf9979209f776)