Avoid Android.mk files in external/exoplayer

This CL:
- Deletes existing redundant Android.mk files.
- Updates the download script to not commit those.

Test: Manually checked output of:
Test: ./download_exoplayer.py --commit=84881739ee80af6c3795bb90a4eae5213e443531
Bug: 199522401
Change-Id: Iba141f5a47d3b9332853a7b3a2f8f1140a91abaa
Merged-In: Iba141f5a47d3b9332853a7b3a2f8f1140a91abaa
(cherry picked from commit 897d621cce58ef9ecfaaebc6e104cd9fb1232ade)
9 files changed