uncrypt: Fix the comparison in FindBlockDevice().

Previously it considered a match if the given path (i.e. path to an
update package) fully equals to a mount_point.  For example, `uncrypt
/data block.map` or `uncrypt /vendor block.map` would exit successfully,
without producing a block map.

Test: `uncrypt /path/to/package.zip block.map`
Test: `uncrypt /vendor block.map` fails.
Change-Id: Id946ab1c0b158b623013f89463cbb1960141d8b5

uncrypt/uncrypt.cpp

@@ -174,9 +174,7 @@ static std::string FindBlockDevice(const std::string& path, bool* encryptable, b
     if (entry.mount_point.empty()) {
       continue;
     }
-    auto len = entry.mount_point.size();
-    if (android::base::StartsWith(path, entry.mount_point) &&
-        (path[len] == '/' || path[len] == 0)) {
+    if (android::base::StartsWith(path, entry.mount_point + "/")) {
       *encrypted = false;
       *encryptable = false;
       if (entry.is_encryptable() || entry.fs_mgr_flags.file_encryption) {