Introduce a new APK attribute: emergencyInstaller

Allow app stores in the system image to designate in their manifest the package name of another system app which can serve as an “emergency installer”.

See more details here: go/android-firebrick-emergency-installer-1pager

Bug: 321080601
Test: m
Change-Id: Ibaac0c7dc2d038e9d08f3543d54ea94178387d15