Import abseil-cpp to third_party

Renamed abseil modules to make them webrtc specific.
Restricted visibility of abseil modules to webrtc packages.

Bug: 171836454
Test: Builds

Change-Id: Icf24c7e3093a77dd9a26e7ec78bd531eb1ad3dec

android_tools/generate_bp.py

@@ -126,7 +126,7 @@ def GenerateDefault(targets):
         print('        "-DWEBRTC_LINUX",')
         print('    ],')
         print('    header_libs: [')
-        print('      "libabsl_headers",')
+        print('      "libwebrtc_absl_headers",')
         print('    ],')
         print('    static_libs: [')
         print('        "libaom",')