Remove complicated previousAppId usage

Currently, previousAppId being < 0, == 0, or > 0 has different meanings,
which is pretty confusing. This commit removes this existing
complication, and will only perform appId migration if and only if a
valid previousAppId argument is passed.

Test: m installd
Bug: 220015249
Change-Id: I0c52dad86c0e8d7442e659b2fad7a95ac610f774