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.go

@@ -0,0 +1,106 @@
+package android
+
+// Split tunnelling modes, stored as strings so an unknown value written by a
+// newer build degrades to "off" rather than to some other mode's behaviour.
+const (
+	SplitTunnelModeOff     = "off"
+	SplitTunnelModeExclude = "exclude"
+	SplitTunnelModeInclude = "include"
+)
+
+type splitTunnelSection struct {
+	Mode     string   `json:"mode"`
+	Excluded []string `json:"excluded"`
+	Included []string `json:"included"`
+}
+
+// PackageList wraps []string for gomobile compatibility.
+type PackageList struct {
+	items []string
+}
+
+// NewPackageList creates an empty list to fill via Add.
+func NewPackageList() *PackageList {
+	return &PackageList{}
+}
+
+// Add appends a package name, ignoring empty ones.
+func (l *PackageList) Add(s string) {
+	if s == "" {
+		return
+	}
+	l.items = append(l.items, s)
+}
+
+// Size returns the number of entries.
+func (l *PackageList) Size() int {
+	return len(l.items)
+}
+
+// Get returns the entry at index i, or an empty string when out of range.
+func (l *PackageList) Get(i int) string {
+	if i < 0 || i >= len(l.items) {
+		return ""
+	}
+	return l.items[i]
+}
+
+// SplitTunnelSettings is one profile's choice of which applications the tunnel
+// carries. The two selections are kept apart because the platform applies one
+// or the other and never both, and so that switching mode does not throw away
+// the picks made in the other one.
+type SplitTunnelSettings struct {
+	Mode     string
+	Excluded *PackageList
+	Included *PackageList
+}
+
+// NewSplitTunnelSettings creates settings that carry every application.
+func NewSplitTunnelSettings() *SplitTunnelSettings {
+	return &SplitTunnelSettings{
+		Mode:     SplitTunnelModeOff,
+		Excluded: NewPackageList(),
+		Included: NewPackageList(),
+	}
+}
+
+func packagesOf(list *PackageList) []string {
+	if list == nil {
+		return nil
+	}
+	out := make([]string, 0, len(list.items))
+	out = append(out, list.items...)
+	return out
+}
+
+func normalizeSplitTunnelMode(mode string) string {
+	switch mode {
+	case SplitTunnelModeExclude, SplitTunnelModeInclude:
+		return mode
+	default:
+		return SplitTunnelModeOff
+	}
+}
+
+func settingsFromSection(section splitTunnelSection) *SplitTunnelSettings {
+	out := NewSplitTunnelSettings()
+	out.Mode = normalizeSplitTunnelMode(section.Mode)
+	for _, pkg := range section.Excluded {
+		out.Excluded.Add(pkg)
+	}
+	for _, pkg := range section.Included {
+		out.Included.Add(pkg)
+	}
+	return out
+}
+
+func sectionFromSettings(settings *SplitTunnelSettings) splitTunnelSection {
+	if settings == nil {
+		settings = NewSplitTunnelSettings()
+	}
+	return splitTunnelSection{
+		Mode:     normalizeSplitTunnelMode(settings.Mode),
+		Excluded: packagesOf(settings.Excluded),
+		Included: packagesOf(settings.Included),
+	}
+}