Fix HLS timeline jump when PROGRAM-DATE-TIME shifts

[AndyWangLYN]

Summary:

• Preserve overlapping segment timing when refreshed HLS playlists report a shifted PROGRAM-DATE-TIME.
• Add a regression test for HLS: Player stuck in infinite buffering loop when #EXT-X-PROGRAM-DATE-TIME changes between playlist refreshes #3337.
• The only intentional behavior change is the bug case: old playlist exists, refreshed playlist has PROGRAM-DATE-TIME, and segments overlap. In that case, we preserve media timeline continuity from the overlapping segment instead of trusting the shifted wall-clock metadata.

Tests:

• ./gradlew :lib-exoplayer-hls:compileDebugJavaWithJavac :lib-exoplayer-hls:compileDebugUnitTestJavaWithJavac
• Not run: DefaultHlsPlaylistTrackerTest requires Java 21 locally via Robolectric SDK 36; this machine has Java 17.