travis: switch to clang-8 for android host build

.travis.yml

@@ -122,11 +122,10 @@ jobs:
       addons:
         apt:
           packages:
-            - clang-9
+            - clang-8
           sources:
-            - sourceline: 'deb https://apt.llvm.org/xenial/ llvm-toolchain-xenial-9 main'
+            - sourceline: 'deb https://apt.llvm.org/xenial/ llvm-toolchain-xenial-8 main'
               key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
-          
       before_script:
         - export ANDROID_HOME=$TRAVIS_BUILD_DIR/sdk
         - sh scripts/travis_common_deps.sh