Support testing Direct Reply with EditTextVariations on API 24/25

This is a follow up CL to my previous CL [1], which enabled us to test
Direct-Reply with EditTextVariations.

What this CL does are:

 * Specyfing min_sdk_version to avoid INSTALL_FAILED_OLDER_SDK error
   when installing on older devices.
 * Avoiding NoSuchMethodError on pre-O devices where
   Notification.Builder does not have a constructor that takes
   notification channel.
 * Fixing a race condition where notification can be sent before
   notification channel is created.

 [1]: Iafffcc7d138b0f502116a5e557f0c3f17e9d0b73
      da2486fd

Bug: 122957841
Test: Made sure that we can install EditTextVariations on N devices
      and "Direct-Reply" on EditTextVariations works there.
Change-Id: Ib4fbd447608b111e763fde4287226cf7e206e65e