releasetools: Fix the bug in symlink deletion.

For file-based OTAs, symlinks in the source build but not in the target
build will be deleted. However, if a symlink is replaced by a regular
file in the target build, the file will be accidentally deleted when
applying (resuming) the same package again.

Verify the checksum of a symlink that will be unpacked or renamed to.
Delete the file only if it doesn't have the target checksum.

Bug: 23646151
Change-Id: I77bae035e39f2e0be25f7f6d71c5882464e3d50f
(cherry picked from commit 84006eac)