Emit gtest in BUILD files only if Android.bp sets it

cc_test bazel macro will default `gtest` to True (similar to Soong). So
we can skip explicitly setting this in the generated BUILD files. The
benefit will be that it will make the generated files less verbose,
without loss of information.

This will be implemented by changing its datatype to *bool from bool.

Test: go test ./bp2build
Test: TH
Change-Id: I284e10f1d58c8e7893b170209827f7d5084ca95e