MemberProfileImage: use defaultProfileIconColor instead of secondaryVariant

MemberProfileImage hard-coded color = MaterialTheme.colors.secondaryVariant
as default, which is LightGray (#F1F2F6) on LIGHT — slightly bluish and
nearly invisible against the ~#F0F0F0 canvas. Reuse the defaultProfileIconColor()
helper so the LIGHT default matches the rest of the app (midpoint between
canvas and white card), and DARK themes keep their palette secondaryVariant.

defaultProfileIconColor() in ChatInfoImage.kt promoted from private to file-
level visibility so it can be referenced from GroupMemberInfoView.

apps/multiplatform/common/src/commonMain/kotlin/chat/simplex/common/views/chat/group/GroupMemberInfoView.kt

@@ -895,7 +895,7 @@ private fun RoleSelectionRow(
 fun MemberProfileImage(
   size: Dp,
   mem: GroupMember,
-  color: Color = MaterialTheme.colors.secondaryVariant,
+  color: Color = defaultProfileIconColor(),
   backgroundColor: Color? = null,
   async: Boolean = false
 ) {

apps/multiplatform/common/src/commonMain/kotlin/chat/simplex/common/views/helpers/ChatInfoImage.kt

@@ -25,7 +25,7 @@ import dev.icerock.moko.resources.ImageResource
 import kotlin.math.max
 
 @Composable
-private fun defaultProfileIconColor(): Color =
+fun defaultProfileIconColor(): Color =
   if (CurrentColors.value.base == DefaultTheme.LIGHT)
     MaterialTheme.colors.background.mixWith(MaterialTheme.colors.onBackground, 0.97f)
   else