AWTPlotRenderer: use float arithmetic instead of double followed by a conversion

Multiplying by 0.9 and converting back to float is slightly more
complicated and achieves the same effect as multiplying by the float
0.9f directly.

Change-Id: I555863bb1251badee8887b9b93c5463eda0a3509
Signed-off-by: Jonathan Nieder <jrn@google.com>