Move /webrtc/api/android files to /webrtc/sdk/android

I decided to make one webrtc/sdk/android/BUILD.gn for both tests and Java/jni src.

External dependencies needs to be updated after this CL.

Future work is required to clean up the Android api and move
implementation details to /webrtc/sdk/android/src.

BUG=webrtc:5882,webrtc:6804
NOPRESUBMIT=True

Review-Url: https://codereview.webrtc.org/2547483003
Cr-Original-Commit-Position: refs/heads/master@{#15443}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 768c64877e8ab88e1864b5017241325f28a3ef34
98 files changed