Flip on Bazel mixed builds by default

This turns on Bazel-assisted builds for all Soong users.

If this CL breaks your build or your workflow, you may temporarily
disable Bazel mode by adding BUILD_BROKEN_DISABLE_BAZEL=1 to your
environment.

Test: TH
Test: Various off-band post-submit verification with "bazel staging
mode", which is an opt-in version of this feature.

Change-Id: Ifa44a79437ce76ea739ed661d4e2d567e0f4a4d5