Make SystemApi retained at runtime

SystemApi annotation is now retained at runtime in order for the
CtsSignatureTest to extract the list of System APIs from the device and
compare it with the known list of the APIs. Until now, the test only
ensured that the known list of the APIs exist in the device, but didn't
ensured the opposite. In order words, it was possible for device
manufacturers to revive the APIs that were once deleted. This will
become a problem when SystemAPIs will be allowed to non-system partitions
such as vendor. An apk in the vendor partition which is using the
revived-but-officially-deleted System API will break when the system
partition is replaced with the Generic System Image (GSI) built from
AOSP, which doesn't have the deleted API. (Running CTS on GSI is part of
the Treble compliance tests)

The overhead of retaining this annotation at runtime is small, because
the relationship from an entity (a class, a field, a member, etc) to the
annotation is recorded as a 4 byte offset field [1].

[1] https://source.android.com/devices/tech/dalvik/dex-format#annotations-directory

Bug: 70832217
Bug: 67891551
Test: measure the size increment before and after. framework.jar was
increased by 2964 byte.

Change-Id: I679b19ac5ce57d33ce59e32b3b1753f8a1962e11