Allow other processes to run snapshotctl through init.

Running snapshotctl involves talking to gsid, which requires the UID to
be 0. To allow other processes to run snapshotctl without running as the
root user, this change adds system properties "sys.snapshotctl.map" and
"sys.snapshotctl.unmap" that proxy snapshotctl calls.

Bug: 311377497
Test: adb shell setprop sys.snapshotctl.map requested
Test: adb shell setprop sys.snapshotctl.unmap requested
Change-Id: Ia29dde30021a94511b8e699c2c8f3816b851bf5c