Extract fsverity_manifest_generator logic

Let the fsverity_manifest_generator logic be reused and just keep the
bits to generate the BuildManifest.apk. Since this can all be acheived
with a series of shell commands, remove the python script and just do it
directly in the Makefile.

Bug: 237384936
Test: TH
Change-Id: I168dc1cea0be72b5098f99e4183e080a687fb03e