[CI] Update android.yml to use protobuf 3.15

Summary:
Update buildkite yaml to use protobuf 3.15 through building it ourselves.

Part of https://linear.app/comm/issue/ENG-2404

Depends on D5861

Test Plan: https://buildkite.com/comm/android-ci-plus-protoc-install/builds/3#018512ba-527e-4f39-9553-7e3d371dee3b

Reviewers: varun, max

Reviewed By: varun, max

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5872