BT: correct abs volume curve for low levels

When BT A2DP absolute volume is used, media volume steps 1,2,3 are
controlled by the framework. This change makes step 1 more useable
by making it slightly louder, and decreases the volume change step
for 2 and 3.

Bug: 176463546
Test: use A2DP device with abs vol, and change media vol to 1/2/3
Change-Id: I7647c89d5d4360feba0d9c4accd6648eececa07a