Add com.android.tools.build:gradle to dependabot gradle-plugin group

[jmagman]

19 PRs (so far) have been opened to update com.android.tools.build:gradle to 8.11.1
https://github.com/flutter/packages/pulls?q=is%3Apr+dependabot+Bump+com.android.tools.build%3Agradle+8.11.1+in%3Atitle+

Instead, group Gradle package dependabot changes into one PR instead of one-per-package.
See #8551 for how this is already being done with org.jetbrains.kotlin:kotlin-gradle-plugin.

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[jmagman]

Alternatively I could break it into its own group instead of including with kotlin-gradle-plugin

      kotlin-gradle-plugin:
        patterns:
          - "org.jetbrains.kotlin:kotlin-gradle-plugin"
      gradle-plugin:
        patterns:
          - "com.android.tools.build:gradle"