Uses placeholder and percentage formatter for seek bar state strings.

Placeholders and percentage formatter are best practice to help prevent
accidental translation errors, especially when mixing formatted strings
with literal percent signs.

Fix: 366201919
Flag: EXEMPT minor string format fix with no functionality change
Test: Use TalkBack to observe the state description of the seekbar;
      observe description is unchanged (e.g. "60% left, 40% right")
Test: atest BalanceSeekBarTest
Change-Id: Ie9dcc9219d253795be31b39279ed9d01d8794f66