Subsume INTEGRATED_BP2BUILD into other env modes

This refactors bazel-build mode determination logic in soong_ui so it's
clearer which of three possible modes are being used in a given
invocation (NO_BAZEL, GENERATE_BUILD_FILES, or MIXED_BUILDS).

Test: bootstrap tests
Change-Id: I41d2baebf8d560c2cc42db8daa8b936101d453e3