Fix outdated docs of permission protection levels.

1) Added deprecation notice to the 'system' and 'signatureOrSystem' protection levels.
These were deprecated in API Level 23:
https://developer.android.com/reference/android/content/pm/PermissionInfo.html#PROTECTION_SIGNATURE_OR_SYSTEM
https://developer.android.com/reference/android/content/pm/PermissionInfo.html#PROTECTION_FLAG_SYSTEM

2) Removed an outdated list of protection levels from PermissionInfo.protectionLevel's JavaDoc.
Its Javadoc has a link to an up-to-date list.

Test: Ran "make docs".
Change-Id: Ia6dc88ba3a66f248f60f010323d5ba40200c1cda