metrics: Tweak RAILMode interpretation
metrics: Tweak RAILMode interpretation

Changes the interpretation of the idle RAIL Mode to background and
raises the priority of load above animation so RAIL Mode animation is
just the default rather than indicating that any actual animation is
happening.

Bug: 172557694
Change-Id: I766f43b2ef4236fb0854236f25769891d63ca2be
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto