Update android build instruction for webrtc unity plugin.

BUG=webrtc:8067

Review-Url: https://codereview.webrtc.org/2993423002
Cr-Original-Commit-Position: refs/heads/master@{#19313}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 3abb73819948535d13beeb654453ade6bc5ffec3
1 file changed