WriteFileRule: Chunk long content and merge them to result

sbox.textproto files are created when handling genrule. The command
for the genrule and output files are written to this file in the
following format:

commands <
  command: "...."
  copy_after: <
    from: "out/asm-generic/auxvec.h"
    to: "out/soong/.intermediates/kernel/msm-4.19/qti_generate_kernel_headers_arm/gen/asm-generic/auxvec.h"
  >
  copy_after: <
    from: "out/asm-generic/bitsperlong.h"
     to:
     "out/soong/.intermediates/kernel/msm-4.19/qti_generate_kernel_headers_arm/gen/asm-generic/bitsperlong.h"
  >
  ....
>

This file grow by one copy_after entry for each output file.

When generating kenrnel headers where the number of output files are
~1000 we run into problems as the contents of sbox.textproto files
are written to disk by generating a shell script using the following
template: /bin/bash -c 'echo -e "$$0" > $out' $content
If $content is very long as in the case of generating kernel headers we
run into the issue where the command line is so long that the shell
script return E2BIG.

Fix this issue by chuking contents into smaller files and then merge
them as a final step.

Test: Build
Issue: 174444967
Change-Id: I1a74023b4222d19672e4df7edb19810a9cf2136f