Avoid writing soong_build_metrics.pb multiple times

Previously, running `m nothing`	would write the	soong_build_metrics.pb
file 3 times:
1. For bp2build - it contains a single `soong_build` event as the rest
   is written to the `bp2build_metrics.pb` file.
2. For Bazel symlink forrest - it too contains a single `soong_build`
   event as the rest is appended to the `bp2build_metrics.pb` file.
3. For soong_build proper (including mixed builds) - it contains useful
   information.

This change avoids that by pushing the code to write the
soong_build_metrics.pb file from the top level where it applies to all
build modes into the build mode specific function that needs it.

Bug: 257590265
Test: m nothing
      # Run with some logging to make sure the file is only created once.
Change-Id: Iee239fb87edf443fed65156fa14b8a30c89a2328

Change-Id: I09724f6143352de6b7d6fb29eaf3fbed5a1abd21