Fix wakelock acquisition & notification strings configuration and usage

[agonper]

This PR is aimed to fix two important issues:

• On certain devices the execution window available for a broadcast receiver before CPU enters into sleep mode is smaller than the standard (10 seconds). Now energy wakelock is acquired earlier, thus overcoming this problem.
• Foreground notification texts were forced to be included in strings.xml even for those applications not executing foreground tasks. Now, notification channel setup functionality skip those channels for which notification texts hadn't been set. In addition, the error message has been improved when trying to create a notification for an unconfigured channel (with missing strings).

[agonper]

Wow, that was fast! Thanks Miguel for having a look at it!