Update core dependencies

[chingor13]

• update gax to 1.48.0
• update google-auth-library to 0.17.0
• update grpc to 1.23.0
• ensure guava versions at 28.0-android
• update protobuf to 3.9.1
• update opencensus to 0.23.0
  • in opencensus 0.22.0, the runningSpanStore defaulted to not storing spans, so we need to configure the store to actually keeps some spans for testing
• update auto-value to 1.6.6

[codecov]

Codecov Report

Merging #6115 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #6115 +/- ##
============================================
+ Coverage 47.38% 47.38% +<.01% - Complexity 27182 27197 +15 
============================================
Files 2523 2523 Lines 274600 274600 Branches 31383 31378 -5 ============================================
+ Hits 130124 130125 +1 + Misses 134863 134862 -1 
Partials 9613 9613

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/google/cloud/pubsub/v1/Publisher.java	89.45% <0%> (+0.34%)	40% <0%> (ø)	⬇️

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[chingor13]

I cannot figure out why this won't compile on Java 7. It's failing to find another package installed from this repo.

[chingor13]

autovalue >= 1.5 must use Java 8 for compilation but will work on Java 7