Resolves transaction codes lazily when a dump is requested.

This change allows to save:
- some memory by removing the method name from CallStat and CallSession
- some CPU by not calling binder.resolveCode for every binder call
at the expense of some hackiness (calling getDefaultTransactionCode
using reflection)

BinderCallsStats#getExportedCallsStats is now more costly to call but it
will be invoked only once a day only.

Test: unit test
Change-Id: Id4f933644c6da9d4aa2d0671db4ab80676eac1b0