Add AHB and Pixel- Formats for R8

Add AHARDWAREBUFFER_FORMAT_R8_UNORM and PIXEL_FORMAT_R_8. These will be
used for an alpha 8 buffer representing the screen decorations.

Enum value matches AIDL R_8 in
I6f6b7f22e76609151a413ca6a6dbf55bd91911dd.

Bug: 193170859
Test: make
Change-Id: I4846327b29736ee811672801a683deb287342a8e