Fix clang-tidy warnings in bt.

Fix clang-tidy warnings in bt.

* Add explicit keyword to conversion constructors,
  or add NOLINT to implicit conversion constructors.
Bug: 28341362
* Use const reference type for read-only parameters,
  or add NOLINT to keep same callback function signature.
Bug: 30407689
* Use const reference type for loop index variables to avoid unnecessary copy.
Bug: 30413223
* Use const reference type to avoid unnecessary copy.
Bug: 30413862
Test: build with WITH_TIDY=1

Change-Id: I8237d332f32d7be09ab72a8de82160fdb776dcd1