Implement general projection curve support for UsageGraph.

Previously, projections were hard-coded in UsageGraph as lines from the
last known point to the corner of the graph. This change replaces that
with support for arbitrary projection curves. Logic for hiding/showing
the projection is now gone; if the client does not want a projection,
it simply does not supply one.

There are two active clients of this code: the data usage graph and the
battery usage graph. The data graph does not use projections and is
essentially unchanged. The battery graph now implements its linear
extrapolation directly in BatteryInfo.

Bug: 38400320
Test: make SettingsUnitTests SettingsGoogleUnitTests
Test: manual (screenshots in comments)
Change-Id: I754e66f6b18ecb8b936143399f8e9e3368fc1ce4