Add a way to create snapshot repository directory

Add a way to build all the androidx libraries where every project
version is replaced with A.B.C-SNAPSHOT.

For these snapshot builds, maven plugin tasks will put their results
to dist/ directory that will make them accessible to users of
ci.android.com

This CL also introduces Publish enum that allows library to show up
in snapshots but not be released.

This will allow developers to point their local build.gradle files to
test out snapshots of our libraries by adding something like:
repositories {
  maven {
    url "https://ci.android.com/builds/submitted/5610982/androidx/latest/view/repository"
  }
}

Test: SNAPSHOT=true ./gradlew createArchive -> creates a repository directory in out/dist
Change-Id: I3e4ef04b59d2e5114f8aeaf6aca5bb0c931c1397

diff --git a/navigation/common/build.gradle b/navigation/common/build.gradle
index fdd02fa..583dbe5 100644
--- a/navigation/common/build.gradle
+++ b/navigation/common/build.gradle

@@ -18,6 +18,7 @@
 import androidx.build.LibraryGroups
 import androidx.build.LibraryVersions
 import androidx.build.AndroidXExtension
+import androidx.build.Publish
 
 plugins {
     id("AndroidXPlugin")
@@ -63,7 +64,7 @@
 
 androidx {
     name = "Android Navigation Common"
-    publish = true
+    publish = Publish.SNAPSHOT_AND_RELEASE
     mavenVersion = LibraryVersions.NAVIGATION
     mavenGroup = LibraryGroups.NAVIGATION
     inceptionYear = "2017"