Some system apps may download unknown content and the user should
be explicitly asked whether they trust these files. System apps should
explicitly use the extra NOT_UNKNOWN_SOURCE to bypass this check.
Test: Builds, boots, existing tests pass:
atest CtsPackageInstallTestCases
Locally verified they pass if CtsPackageInstallTestCases.apk was signed by
the platform cert.
Bug: 123700348
Change-Id: I3028bf8ff3f79a41521deeee43fba3c32bb1b2ca
Merged-In: I2578251906f6656b83464d1c4fc4db99165841c9
43e682abef