minadbd: `adb rescue getprop` returns newline-terminated result.

This change addresses the comment in [1], which makes the results of
`adb shell getprop foo` and `adb rescue getprop foo` more consistent.
That is, both will return newline-terminated results now.

[1] https://r.android.com/c/platform/bootable/recovery/+/976340/3/minadbd/minadbd_services.cpp#188

Test: Run the following commands on taimen (under rescue mode):
    `adb rescue getprop ro.build.fingerprint`
    `adb rescue getprop ro.nonexistent`
    `adb rescue getprop`
Change-Id: I5af47f8ea4d569b8507e259daef87749c0945f47