Add @JvmField annotation to flags

Adding @JvmField annotation to flags that have @Keep defined.
Without it, optimizer seems to remove them.

Bug: 256020545
Test: build sysui from studio and makepush, observe that updated flags
are visible in Flag Flipper app for both cases

Change-Id: Ibec4dbe6a54f881acf7f3a4f063dd7e512bb9a90