AW: Add AndroidXPlugin in support boundary interfaces gradle file

Applying AndroidXPlugin to fix issues with signing project builds when
it's mirrored to AndroidX repo.

Bug: https://issuetracker.google.com/issues/142394338
Test: manually cherry-pick change to //external and run gradlew :webkit:assembleRelease and gradlew :webview-support-interfaces:assembleRelease
Change-Id: I4156549137c43128087b94307d14284d5b08414e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1853265
Auto-Submit: Hazem Ashmawy <hazems@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Aurimas Liutikas <aurimas@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#704758}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 96b1763c14c5ffcb0700a203123218b9aa28e1ee
diff --git a/build.gradle b/build.gradle
index 300ab6c..6bf1a27 100644
--- a/build.gradle
+++ b/build.gradle
@@ -8,6 +8,7 @@
 import androidx.build.SupportConfig
 
 plugins {
+    id('AndroidXPlugin')
     id('com.android.library')
 }