Build dynamic iOS SDK.

- Places most ObjC code into webrtc/sdk/objc instead.
- New gyp targets to build, strip and export symbols for dylib.
- Removes old script used to generate dylib.

BUG=

Review URL: https://codereview.webrtc.org/1903663002

Cr-Commit-Position: refs/heads/master@{#12524}

PRESUBMIT.py

@@ -229,6 +229,7 @@ def _CheckNoRtcBaseDeps(input_api, gyp_files, output_api):
         'base_tests.gyp',
         'desktop_capture.gypi',
         'p2p.gyp',
+        'sdk.gyp',
         'webrtc_test_common.gyp',
         'webrtc_tests.gypi',
     )