Unconditionally install shim apex on system partition

Trying to do it on per-device basis is prone to errors and already
bitten us several times. For example, currently aosp_taimen doesn't
install shim apex on system partition, but specifies
ro.apex.updatable = "true", which means that it doesn't pass CTS tests.

Unconditionally installing shim APEX shouldn't introduce any problems
since apexd will skip its activation of devices that don't support
updatable APEX.

Test: m checkbuild
Bug: 140957666
Change-Id: I6b5e668b40b97752295c831684a7291842533c40