Store the Android split tunnelling settings per profile (#7349)

Which applications the tunnel carries belongs with the rest of a
profile's preferences rather than on the Android side, so the choice
follows the profile the user is on.

Adds a split tunnel store beside the SSH session store, over a
"split-tunnel" namespace holding the mode and both selections. The two
selections are kept apart because the platform applies an allow list or
a deny list and never both, and so that switching mode does not throw
away the picks made in the other one.

Only the store itself needs the android build tag; the rest stays
untagged so it is covered by the package's host tests.

client/android/split_tunnel_store.go

@@ -0,0 +1,34 @@
+//go:build android
+
+package android
+
+const splitTunnelNamespace = "split-tunnel"
+
+// SplitTunnelStore reads and writes a profile's split tunnelling settings.
+type SplitTunnelStore struct {
+	prefs prefsStore
+}
+
+// NewSplitTunnelStore opens the split tunnelling store of the given profile.
+func NewSplitTunnelStore(configDir, profileID string) (*SplitTunnelStore, error) {
+	prefs, err := newProfilePrefs(configDir, profileID)
+	if err != nil {
+		return nil, err
+	}
+	return &SplitTunnelStore{prefs: prefs}, nil
+}
+
+// Load returns the stored settings, or settings that carry every application
+// when the profile has none saved.
+func (s *SplitTunnelStore) Load() (*SplitTunnelSettings, error) {
+	var section splitTunnelSection
+	if _, err := s.prefs.Get(splitTunnelNamespace, &section); err != nil {
+		return nil, err
+	}
+	return settingsFromSection(section), nil
+}
+
+// Save replaces the stored settings.
+func (s *SplitTunnelStore) Save(settings *SplitTunnelSettings) error {
+	return s.prefs.Put(splitTunnelNamespace, sectionFromSettings(settings))
+}