OP_REQUEST_INSTALL_PACKAGES denied by default

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: I2578251906f6656b83464d1c4fc4db99165841c9