Removed nano proto accessors usage

The generated proto java files consists 484 methods which
might break DEX's 64k method limit. Removed the accessors and
directly accessing the fields can lower the generated method
counts to 140. This will require some changes on the server
side as well.

Test: runtest -x frameworks/opt/telephony/tests/telephonytests/src/
com/android/internal/telephony/metrics/TelephonyMetricsTest.java

bug: 32544494
Change-Id: I766bf3402a138cf0570d5374ed0d8e1d95f25d52