bp2build/b: exit early in GENERATE_BAZEL_FILES=1.

This CL fixes a typo in writeFakeNinjaFile to correctly write
a fake out/soong/build.ninja and its depfile. It also modifies bootstrap
phase to *not* run the main soong build phase (which takes more than a
minute) if GENERATE_BAZEL_FILES=1.

This change has the side effect that `GENERATE_BAZEL_FILES=1 m nothing`
no longer generates the real build.ninja, which is fine because one
shouldn't be using GENERATE_BAZEL_FILES=1 for that anyway (or, use
USE_BAZEL_ANALYSIS=1). This change has no effect on mixed builds.

Time on a change to Soong or any Android.bp files:

Before: bp2build_workspace_marker (~20 seconds) + build.ninja (1 min)
After: bp2build_workspace_marker (~20 seconds)

Time on the second of two consecutive `b build //bionic/...`: 2.070s

Test: TH
Test: Soong integration tests
Change-Id: I43720641815994caba97b8d165d7c3fc254cbd06