deps: update dependency com.google.guava:guava-bom to v29 (#205)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [com.google.guava:guava-bom](https://github.com/google/guava) | major | `28.2-android` -> `29.0-android` |

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pom.xml

@@ -164,7 +164,7 @@
     <protobuf.version>3.11.4</protobuf.version>
     <opencensus.version>0.24.0</opencensus.version>
     <annotations-api.version>1.3.2</annotations-api.version>
-    <guava.version>28.2-android</guava.version><!-- We are currently using the *-android version to support JDK7. -->
+    <guava.version>29.0-android</guava.version><!-- We are currently using the *-android version to support JDK7. -->
     <junit.version>4.13</junit.version>
     <truth.version>1.0.1</truth.version>
     <easymock.version>3.6</easymock.version>