Audio HAL HIDL wrapper: Fix incorrect conversion of TTY Mode

The TTYMode enum numeric value was converted to a char* instead of its
literal value and vice versa.

Instead convert it to/from its literal value.
Eg: TTYMode::FULL -> "FULL" instead of "3"
    "FULL" -> 0 as atoi conversion would fail

Test: atest VtsHalAudioV4_0TargetTest
Bug: 71486871
Change-Id: I29bbf6bf3b5532269afcc5d39ea10eff2871bdea
Merged-In: I29bbf6bf3b5532269afcc5d39ea10eff2871bdea
Signed-off-by: Kevin Rocard <krocard@google.com>
Signed-off-by: Jungyee Yoo <jungyee.yoo@lge.corp-partner.google.com>