fix: Add fallback for contentBitrate and convert adBitrate from KBPS to BPS

[avinash-newrelic]

Summary

• contentBitrate was always reported as 0 when manifest-declared bitrate was unavailable. Added a fallback to calculate bitrate from actual download data.
• adBitrate was incorrectly reported in KBPS instead of BPS. Applied conversion (×1000) to match the data model specification.

Changes

• NRTrackerExoPlayer.java — Added fallback bitrate calculation using download speed when player.getVideoFormat().bitrate is unavailable
• NRTrackerIMA.java — Multiplied getVastMediaBitrate() by 1000 to convert KBPS to BPS

Test Plan

• Play content video and verify contentBitrate reports a non-zero value in BPS
• Play content with HLS/DASH stream and verify manifest-declared bitrate is still preferred
• Play video with IMA ads and verify adBitrate reports in BPS (e.g., 500000 instead of 500)
• Verify cross-platform consistency with iOS bitrate values