adb: use /system/bin/remount command

Replace direct logic in support for 'adb remount' with an exec out to
/system/bin/remount to do the heavy lifting.  Remount success and
failure strings are reported by the adb remount service in response
to the various reported errors, freeing up the remount command itself
from the legacy of script expectations.

Test: adb-remount-test.sh
Bug: 122602260
Change-Id: I686fa465f463b881bbb38f709d780a95e463be80