Freezer: add flag for INSTALL_PACKAGE exemption

If activity_manager_native_boot.freeze_exempt_inst_pkg is true, exempt
apps with INSTALL_PACKAGE permission from being frozen.

Otherwise, disable this exemption so apps with INSTALL_PACKAGE can also
be frozen.

Bug: 253913470
Test: verify com.android.vending can be frozen in cached mode.
Change-Id: I6f54edb3f7f469bfd242e6306a1fd1192f1df17a