Bump protobuf-java from 3.11.4 to 3.15.6

Bumps [protobuf-java](https://github.com/protocolbuffers/protobuf) from 3.11.4 to 3.15.6.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.11.4...v3.15.6)

Signed-off-by: dependabot[bot] <support@github.com>

library/build.gradle

@@ -31,7 +31,7 @@ dependencies {
     api 'com.google.flogger:flogger-system-backend:0.5.1'
     api 'com.google.code.findbugs:jsr305:3.0.2'
     api 'com.google.guava:guava:30.1.1-android'
-    api 'com.google.protobuf:protobuf-java:3.11.4'
+    api 'com.google.protobuf:protobuf-java:3.15.6'
 
     // CameraX core library
     def camerax_version = "1.0.0-rc04"