deps: update guava to 28.1-android (#32)

Author: chingor13

pom.xml

@@ -163,7 +163,7 @@
     <protobuf.version>3.10.0</protobuf.version>
     <opencensus.version>0.24.0</opencensus.version>
     <annotations-api.version>1.3.2</annotations-api.version>
-    <guava.version>28.0-android</guava.version><!-- We are currently using the *-android version to support JDK7. -->
+    <guava.version>28.1-android</guava.version><!-- We are currently using the *-android version to support JDK7. -->
     <junit.version>4.12</junit.version>
     <truth.version>1.0</truth.version>
     <easymock.version>3.6</easymock.version>