Downgrade to compileSdkVersion 26

Summary:
Go back to using compileSdkVersion 26 and targetSdkVersion 26, temporarily. We can re-add this once Android SDK 27 becomes available in Facebook's internal repository.

The Android SDK Build Tools 27.0.3 **are** available, so we can continue using those.

Reviewed By: axe-fb

Differential Revision: D9886607

fbshipit-source-id: 6c1c9c1e1309c3a0483cc4c0bd8dcb4a5f29fc7e

local-cli/templates/HelloWorld/android/build.gradle

@@ -4,8 +4,8 @@ buildscript {
     ext {
         buildToolsVersion = "27.0.3"
         minSdkVersion = 16
-        compileSdkVersion = 27
-        targetSdkVersion = 27
+        compileSdkVersion = 26
+        targetSdkVersion = 26
         supportLibVersion = "27.1.1"
     }
     repositories {