adb-remount-test: Add LOG() function

Right now there are a lot of log commands in the form of
  "echo <color code><log type><color code> [msg]... >&2"
which is painful to read, and test writers often accidentally omit the
trailing ">&2".

Add a LOG() function which takes care of the log formatting and stderr
redirecting once and for all.
Also bulk edit existing log commands to use LOG() everywhere.

Bug: 243116800
Test: adb-remount-test --color
Change-Id: I04beb9e09b28c08a3a6f4309bf2d4b6de906df90