User search: provide 10 results not 5

Also refactor slightly so the caller specifies the number of
results, which lets all the constants sit in the repository.

libraries/usersearch/api/src/main/kotlin/io/element/android/libraries/usersearch/api/UserListDataSource.kt

@@ -21,6 +21,6 @@ import io.element.android.libraries.matrix.api.user.MatrixUser
 
 interface UserListDataSource {
     //TODO should probably have a flow
-    suspend fun search(query: String): List<MatrixUser>
+    suspend fun search(query: String, count: Long): List<MatrixUser>
     suspend fun getProfile(userId: UserId): MatrixUser?
 }