Add flagging libraries for trade-in mode.

This is needed to start adding tradeinmode APIs. The flags are getting
split out separately so they can also be used in AOSP for adbd.

Bug: 307713521
Test: builds
Flag: com.android.tradeinmode.flags.enable_trade_in_mode
Change-Id: Ie6db1e093ee4fc0aa4b01bd4e0639a0ceac50e5a
Merged-In: Ie6db1e093ee4fc0aa4b01bd4e0639a0ceac50e5a