releasetools: Fix the tag replacement for ro.build.vendor.fingerprint.

For devices using derived fingerprint (i.e. /system/build.prop doesn't
contain ro.build.fingerprint, but has ro.build.thumbprint instead), the
current code (in android.os.Build) doesn't have a matching logic to do
the same for ro.vendor.build.fingerprint. This means we will see
ro.build.thumbprint in /system/build.prop, while there's no matching
ro.vendor.build.thumbprint in /vendor/build.prop.

From signing script point of view, it should just apply the tag
replacement (e.g. test-keys -> release-keys) for whatever it sees when
signing a target_files.zip.

This CL also adds unit tests for EditTags() and RewriteProps().

Fixes: 27950003
Test: Use 'sign_target_files_apks.py' to sign a target that uses derived
      fingerprint and vendor partition. Check VENDOR/build.prop.
Test: python -m unittest test_sign_target_files_apks
Change-Id: I09019da970840cd82f54b68a32b4e94984bc1d8d