Change opacity of Output Switcher disabled items to meet a11y standards.

Bumping the light theme opacity to 0.6 to pass the contrast
requirement of 3.0:1 for a disabled item text.

Fix: 431259291
Test: atest MediaOutputAdapterScreenshotTest
Test: On a physical device
Flag: EXEMPT BUGFIX
Change-Id: I34b197a26afa6e2cfa2a4733b0e754f6f755ff98