KeyMaster 4.0 VTS: fix GSI detection

Commit f18a8328 ("keymaster: Relax testing under GSI") disabled
some tag checks for devices running with GSI, but detected GSI by
looking for an absence of the ro.boot.vbmeta.device_state property.

This property is currently present on GSI, so instead detect GSI using
the existing is_gsi() helper, which checks ro.product.system.name
against "mainline".

Bug: 192513934
Test: atest VtsHalKeymasterV4_0TargetTest:PerInstance/AttestationTest
Merged-In: If3c7d84a9e091b9b0842e4d8919453600bc239ea
Change-Id: If3c7d84a9e091b9b0842e4d8919453600bc239ea