CB-12802: (android) Fix build.gradle to sign when building with multiple APK is enable.

[kevin-lot]

When 'cdvBuildMultipleApks' is true, 'task.name' can have 'validateSigningArmv7Release' or 'validateSigningX86Release' values.

Platforms affected

cordova 7
cordova-android 6.2.3

What does this PR do?

Fix build.gradle to sign when building with multiple APK is enable.

What testing has been done on this change?

Test to build with or without signature and with or without multiple APK.

Checklist

• Reported an issue in the JIRA database issue on Jira
• Commit message follows the format: "CB-3232: (android) Fix bug with resolving file paths", where CB-xxxx is the JIRA ID & "android" is the platform affected.
• Added automated test coverage as appropriate for this change.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@6fa16b6). Click here to learn what that means.
The diff coverage is n/a.

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  Branches          ?      307           
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  Hits              ?      724           
  Misses            ?      964           
  Partials          ?        0

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