Move 'remount' back into 'adbd_system_api'

It was inlined in base_system.mk during the adbd apex bring up phase,
because adbd apex was thought to have broken "required:" dependency.
Now that we have 'adb_system_api' to track the dependencies of adbd, we
can move 'remount' into the phony module.

Bug: 241688845
Test: m adbd_system_api w/ user & userdebug build
Change-Id: I5d7c66c517d5e7022601cc4705a447d5a4ba1be4