Generate ninja_inputs.pb as build metrics

To monitor ninja inputs, generate ninja_inputs.pb as build metrics.
This will count the number of input files and required git projects,
with the number of files for each project.

Bug: 241033491
Test: $ cp .repo/manifests/default.xml out/dist/manifest_1.xml
      $ development/multitree/build_vendor.sh -d vsoc_x86_64 \
          -p cf_x86_64_phone -r out/dist -i 1
Change-Id: Ie185863f1c8fca313baba6135374d509e2d93937
5 files changed