wallet, rpc: Only allow keypool import from single key descriptors

Legacy wallets should only import keys to the keypool if they came in a
single key descriptor. Instead of relying on assumptions about the
descriptor based on how many pubkeys show up after expanding the
descriptor, explicitly mark descriptors as being single key type and use
that for the check.

src/wallet/test/walletload_tests.cpp

@@ -26,6 +26,7 @@ public:
     bool IsRange() const override { return false; }
     bool IsSolvable() const override { return false; }
     bool IsSingleType() const override { return true; }
+    bool IsSingleKey() const override { return true; }
     bool ToPrivateString(const SigningProvider& provider, std::string& out) const override { return false; }
     bool ToNormalizedString(const SigningProvider& provider, std::string& out, const DescriptorCache* cache = nullptr) const override { return false; }
     bool Expand(int pos, const SigningProvider& provider, std::vector<CScript>& output_scripts, FlatSigningProvider& out, DescriptorCache* write_cache = nullptr) const override { return false; };