Integrate bazelenv.sh environment variables into soong_ui environment.

This removes the need to source bazelenv.sh for USE_BAZEL_ANALYSIS, and
unifies mixed builds to use the checked in tools/bazel and bazelrc.

It also unifies all bazel-related output to be in out/bazel.

Without aosp/1502095, this change still requires toplevel_output_directories to be an empty
list, otherwise there'll be this error:

ERROR: Directories specified with toplevel_output_directories should be
ignored and can not be used as sources.

Test: With aosp/1441774: rm -rf out/ && lunch aosp_cf_x86_auto && USE_BAZEL_ANALYSIS=1 m libc && prebuilts/build-tools/linux-x86/bin/ninja -f out/combined-aosp_cf_x86_auto.ninja -t commands libc | grep bazel-out | wc -l # 2 results
Change-Id: I69b217ec88da531415792bb6e04b6a194ca4718d