Remove the PackageManager.INSTALL_UNKNOWN check (3/n)

Though unlikely, the notification will never be sent and the install
process will stuck forever if one of params.mRet equlas
PackageManager.INSTALL_UNKNOWN.

We should just treat PackageManager.INSTALL_UNKNOWN as failure.

Bug: 203026561
Test: atest CtsStagedInstallHostTestCases

Change-Id: Ib7070ce89a667bf8a5ab2ceed6b0cc4c7fac44c9