Spinner background fixes

We now use the platform Spinner.Underlined style on v23+. We only
have to workaround the RTL bug on v21-22 (which we already do).

Ideally we would use the new vector Spinner assets pre-v23, but
padding and gravity was only added to LayerDrawable in v23.

BUG: 26630588

Change-Id: Idc1acef7c3ab428488d0b692053f2ff2f3c95ecb
2 files changed