Add groundwork to easily all-enable bazel-mode

This will make it easy to change a "false" boolean to "true" to push
`--bazel-mode` by default for all users.

Users may disable bazel-by-default with BUILD_BROKEN_DISABLE_BAZEL, and
bazel is disabled by default on Darwin hosts (due to lack of test
coverage).

Bug: 254628592
Bug: 254084584
Test: Presubmits
Test: Presubmits for aosp/2263623
Change-Id: Iec712119e06ca8ec93028207d88277f879184cc2