Generate placeholder avatars from two characters, re-fetch missed avatars (#856)

* feat: splitting names in the avatar generation

* fix: re-fetch avatars if initial downloads fail

* fix: remove shadowed name, add tests for common labels

app/src/test/java/org/thoughtcrime/securesms/recipients/AvatarGeneratorTest.kt

@@ -0,0 +1,26 @@
+package org.thoughtcrime.securesms.recipients
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+import org.thoughtcrime.securesms.util.AvatarPlaceholderGenerator
+
+class AvatarGeneratorTest {
+
+    @Test
+    fun testCommonAvatarFormats() {
+        val testNamesAndResults = mapOf(
+            "H  " to "H",
+            "Test Name" to "TN",
+            "test name" to "TN",
+            "howdy  partner" to "HP",
+            "testname" to "TE", //
+            "05aaapubkey" to "A", // pubkey values only return first non-05 character
+            "Test" to "TE"
+        )
+        testNamesAndResults.forEach { (test, expected) ->
+            val processed = AvatarPlaceholderGenerator.extractLabel(test)
+            assertEquals(expected, processed)
+        }
+    }
+
+}