Don't override targetVers with minCode

If a package contains minSdkVersion="Q" targetSdkVersion="25",
targetCode will be initialized to "Q" when reading minCode,
but targetVers will be set to "25".  targetCode overrides
targetVers, so this results in the computed targetSdkVersion
being "Q".  If minSdkVersion were instead "28", the computed
targetSdkVersion would be "Q".

Make the computed targetSdkVersion consistent by leaving
targetCode and targetVers unset while parsing minSdkVersion,
and then setting them to minCode and minVers if there is
no targetSdkVersion attribute.

Bug: 110167203
Bug: 110353795
Test: install DeviceHealthChecks, verify targetSdk=25
Change-Id: I9547e9b4720543f0c892cbf4de92888c8eead44f