Bug 1798604 - ThirdPartyPaths.txt has no correct exoplayer2 path. r=andi

Third-party's exoplayer2 path is changed to mobile/android/exoplayer2 by
bug 1745246. So we should update ThirdPartyPaths.txt too.

Differential Revision: https://phabricator.services.mozilla.com/D160999

tools/rewriting/ThirdPartyPaths.txt

@@ -128,7 +128,7 @@ media/openmax_il/
 media/webrtc/signaling/gtest/MockCall.h
 mfbt/double-conversion/double-conversion/
 mfbt/lz4/.*
-mobile/android/geckoview/src/thirdparty/
+mobile/android/exoplayer2/
 modules/brotli/
 modules/fdlibm/
 modules/freetype2/