Refactor: break fs-verity data gen into 2 parts

Break fs-verity generation method into Merkle tree generation and
fs-verity footer (was called header + extension) generation.  This makes
them easier to reuse later.

Also:
 * Add some null-ness annotations.
 * Move generateApkVerity from ApkSigningBlockUtils to ApkVerityBuilder,
   since it's more related to verity but not APK signing format.
 * Rename some of "fs-verity" to "apk-verity" to disambiguate between
   our experimental implementation (deprecated) and upstream.

Test: atest PtsApkVerityTestCases
Bug: 112037636

Change-Id: I0d70121071a3e78f80ae059a60e193afbadea337