Touch app.gradle file only if __PACKAGE__ placeholder exists

[Fatme]

In case when tns run android command is executed, the app is restarted twice on device. The first restart is when the fullSync is executed and the second one is when app.gradle file is interpolated. Actually once the __PACKAGE__ placeholder is replaced inside app.gradle, there is no need to replace it again. This PR skips the interpolation of app.gradle file when there is no placeholder inside it.

Steps to reproduce:

1. tns create myApp
2. tns run android

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• All existing tests are passing: https://github.com/NativeScript/nativescript-cli/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• Tests for the changes are included.

What is the current behavior?

The application is restarted twice on device.

What is the new behavior?

The application is restarted only once on device.