releasetools: Consolidate the codes that generate metadata dict.

Now they (A/B, non-A/B full and non-A/B incremental) share the same
code. Also add tests for the newly added function.

Test: python -m unittest test_ota_from_target_files
Test: Build A/B and non-A/B packages respectively.
Change-Id: I5499dd9c683ac1a18d8a24f21ff8845fe69f343e