Apply fixes for EfficientStringsChecker.

The recently-built Error Prone checker has found many instances where
we're always paying the cost of StringBuilder concatenation, even in
the typical cases where preconditions are successfully met.

Benchmarks have shown that even when replacing these with varargs
formatter strings, the default case is 20x faster.

Bug: 170978902
Test: none
Exempt-From-Owner-Approval: trivial refactoring
Change-Id: If8c00bc73467bfb91ec16c162969c9d26ca53646