Add minTargetSdkVersion input selector to seapp_contexts

This new input selector allows phasing in new security policies by
giving app developers an opportunity to make any needed compatibility
changes before updating each app's targetSdkVersion.

When all else is equal, matching entries with higher
minTargetSdkVersion= values are preferred over entries with lower
minTargetSdkVersion= values.

Test: Marlin builds and boots. Apps targeting targetSdkVersion<=25
run in untrusted_app_25 domain. Apps targeting the current development
build >=26 run in the untrusted_app domain with fewer permissions. No
new denials observed during testing.
Bug: 34115651
Change-Id: I14bf4f51dbe26cb9bd3f62ad0b281085441d9806