InstallerService

This merge request mainly introduces the InstallerService. Many files have been touched and reworked in this merge request, due to the following changes:

• After download of an apk in InstallManagerService, the InstallerServiceis started an kicks off the installation process. For unattended installers this directly runs through without any user interaction, for the default installer a new PendingIntent containing DefaultActivityInstaller is returned that is either stuffed into the notification or directly started from AppDetails
• Using local broadcasts, InstallManagerService and AppDetails are informed of state changes in the installation process
• DefaultActivityInstaller is a wrapper around the default installation APIs of Android
• If the unattended PrivilegedInstaller is available, a permission screen is shown before download
• Actual error codes and messages are displayed in notification or dialog on fail, especially interesting when using the PrivilegedInstaller
• The process for installing the Privileged Extension has been moved into an own installer for logic seperation, called ExtensionInstaller

Some design considerations:

• I try to use Uris where ever possible. At some points this clashes with the usage of urlString in InstallManagerService. This could be fixed in a later merge request

Some other TODOs are left, but I would like to do them after this merge request has been merged if it's okay, as this one is already too huge:

• Check if apk permissions are the same as announced in the permission screen for PrivilegedInstaller
• In Installer.newPermissionCount(), I need the target SDK before download to check if it's targetting Android M, which does not require the permission screen
• Introduce FileProvider for Android N
• Redesign layout of InstallConfirmActivity
• Remove "cancel" icon for installing progress in AppDetails