[PM] Check unknown sources user restriction for intent installation

Bypass the unknown source user restrictions check when either of the
following two conditions is met:
1. the installation is not triggered via ACTION_VIEW or
   ACTION_INSTALL_PACKAGE
2. the value of the Intent.EXTRA_NOT_UNKNOWN_SOURCE is TRUE and the
   caller is a privileged app

Flag: EXEMPT BUGFIX
Bug: 438352252
Test: manual
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:93f16fe40c18d12d83a547408194ff2b1ec4b728
Merged-In: Id5e0eb91d523826161cf4b42f9fdf2c0d5454600
Change-Id: Id5e0eb91d523826161cf4b42f9fdf2c0d5454600