Support generating fs-verity descriptor

Note that since fsverity.h is not yet in bionic, most of them JNI code
is not compiled since HAS_FSVERITY is 0 by default.  The plan is to
remove it once the file is in.

Test: Manually copy linux/fsverity.h to bionic/libc/kernel/uapi, build
      with ENABLE_FSVERITY={0,1}.
Test: atest PtsApkVerityTestCases
Test: With other changes, observe the hash matches what other tool
      generates.
Bug: 112037636

Change-Id: I5e0ef0625a88326203ddfb3cbb73e12f2111b4b8