[client] Reuse the profile's account for Android SSO logins

The Android binding never recorded which account a profile belongs to, so
every interactive login and every session extend went to the IdP with no
login_hint. With nothing to go on the IdP picks an account itself, which on a
session extend means re-authenticating an account the profile is already
signed in with.

Store the email the PKCE flow already parses out of the ID token, and pass it
back as the hint on later flows. An empty hint stays meaningful: a fresh
profile, or one that was logged out, deliberately leaves the choice to the
IdP, which is how a profile changes accounts. Logout clears the stored email
for that reason — while it is on disk it would steer the next login straight
back into the account just logged out of.

The email is keyed off the profile's config path rather than the active
profile: Auth.login runs in a goroutine, so the active profile can change
under a flow already in flight. It lands in <profile>.account.json, not the
<profile>.state.json desktop uses for the same data — there the email and the
engine's state manager sit in different directories, but on Android both
resolve under files/, and the state manager rewrites the whole file from its
own keys.

client/android/session.go

@@ -293,7 +293,10 @@ func (c *Client) extendAuthSession(ctx context.Context, urlOpener URLOpener, isA
 	}
 	defer authClient.Close()
 
-	a := &Auth{ctx: ctx, config: cfg}
+	// Passing the config path makes the flow pick up the login_hint: an extend
+	// renews the session of the account already signed in, so it must not stop to
+	// offer a choice.
+	a := NewAuthWithConfig(ctx, cfg, c.configPathSnapshot())
 	tokenInfo, err := a.foregroundGetTokenInfo(authClient, urlOpener, isAndroidTV)
 	if err != nil {
 		return fmt.Errorf("interactive sso login failed: %v", err)