travis: Install the newer Android SDK we've just switched to.

Oops.  Very similar to 18a59ef.  Add a note to try to avoid this
failure next time.

Author: gnprice

.travis.yml

@@ -15,7 +15,7 @@ android:
     - tools
     - platform-tools
     - build-tools-28.0.3
-    - android-28  # corresponds to compileSdkVersion
+    - android-29  # corresponds to compileSdkVersion
     - extra-google-m2repository
     - extra-android-m2repository
     - extra-google-android-support

android/build.gradle

@@ -19,6 +19,7 @@ buildscript {
         //   https://medium.com/androiddevelopers/picking-your-compilesdkversion-minsdkversion-targetsdkversion-a098a0341ebd
         // What's the latest?  Consult this list:
         //   https://developer.android.com/studio/releases/platforms
+        // Do edit .travis.yml when updating this.
         compileSdkVersion = 29
 
         supportLibVersion = "28.0.0"