Cert: Generate backing process code coverage report after test run

* Generate line-by-line coverage report after test run using
  llvm-profdata and llvm-cov in JSON format
* Iteratively merge coverage report to the same file for the backing
  processes of both cert stack and stack under test even when they
  are using the same binary
* Since we cannot run a method after all tests, we have to iteratively
  merge coverage report into a common file after each test run
* llvm-cov and llvm-profdata tool is packaged together with the test
  ZIP as they have to be the same version when indexing .profraw data

Outputs:

* Two indexed data files per test run:
  - GdDevice_cert_stock_backing_process_coverage.profdata
  - GdDevice_stack_under_test_backing_process_coverage.profdata
* Two reports per test run:
  - GdDevice_cert_stock_backing_process_coverage.json
  - GdDevice_stack_under_test_backing_process_coverage.json
* Two summaries per test run:
  - GdDevice_cert_stock_backing_process_coverage_summary.txt
  - GdDevice_stack_under_test_backing_process_coverage_summary.txt
* Two raw data files per test class
  - GdDevice_cert_stock_backing_process_coverage.profraw
  - GdDevice_stack_under_test_backing_process_coverage.profraw

Performance Analysis:

* File Sizes:
  - profraw: around 6MB, 2 generated per test class
  - profdata: around 400KB, 2 generated per test class
  - coverage report: around 40MB, 2 generated per test run
* Each coverage report takes about 2 seconds and hence 4 seconds
  per test class. 11 test classes would result in 44 seconds for
  the whole test suite
* Hence, additional 80MB storage and about 50 seconds run time per
  test run

Bug: 153384476
Test: gd/cert/run --host
Change-Id: If488f1d1816fb1f9379729ace9d2b0e1a0343233