feat: set min required compile Android SDK to 28

[rosen-vladimirov]

As we are going to set min required Android Support Library to be 28 (in the modules), we need to set the min required compile SDK to 28 as well (as Android Support Library 28 relies on resources included in this version of the SDK and the project cannot be build without them). This way CLI will not allow building an application without installing the Android SDK 28 first.

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What is the current behavior?

CLI allows you to build applications when Android SDK 28 is not installed. Using next of tns-core-modules and trying to build it will fail with some missing resources error.

What is the new behavior?

CLI does not allow you to build applications in case Android SDK 28 is not installed.
Implements issue #3991

BREAKING CHANGES:
You will not be able to build applications for Android without installing SDK 28

Migration steps:
Install Android SDK 28 and build tools 28:

$ANDROID_HOME/tools/bin/sdkmanager "build-tools;28.0.1"
$ANDROID_HOME/tools/bin/sdkmanager "platforms;android-28"