Check discriminator before accessing FrontendCapabilities information.

FrontendCapabilities field is safe_union. Before accessing it, use
discriminator to check if the field is filled or not.

Bug: 190734407
Fix: 190734407
Test: atest android.media.tv.tuner.cts.TunerTest
Change-Id: I744ededd2dfb775783c53279105951d8fdc028f7
1 file changed