Installer (and uninstall) fallback fixes for API >= 24

Installer: Use content URI on API >= 24 for DefaultInstaller fallback

• When there's a permission mismatch (#951, #890), the fallback DefaultInstaller is invoked, which enforces file and content schemes for API < 24 and >= 24 respectively.
• Use content URI in that case, which allows the fallback to work.

PrivilegedInstaller: fallback to default installer for uninstall

• On API >= 24, in cases when the installer package name is not set to privext, the system won't let us uninstall.
• Fallback to the DefaultInstaller so that uninstall still works.

mentioned in issue #951

This could potentially be cherry-picked to v0.102 , it cherry-picks cleanly.

mentioned in merge request !483 (closed)

changed milestone to %0.103 - UX Overhaul

added bazaar bug privileged installer labels

changed title from Installer: Use content URI on API >= 24 for DefaultInstaller fallback to Installer (and uninstall) fallback fixes for API >= 24

What else throws a SecurityException? We wouldn't want to have F-Droid try to do the install again via the other API if there was some other kind of security issue besides the permissions mismatch.

The SecurityException checking here is done only for the setInstallerPackageName command, not the install / uninstall part.

However that method does throw SecurityExceptions for more than just cert mismatch, so maybe we could have a regex matching the cert mismatch message.

merged

mentioned in merge request !486 (merged)