Introduce libshaders static library

This library is for exposing system-only shader implementations in SkSL.
Notably: libshaders depends on the vendor available libtonemap, but
libtonemap is vendor available as it is a single source of truth for
tonemap operations, whereas libshaders is not meant to be shared with
the vendor partition. Rather, the intent is to allow for sharing of SkSL
between librenderengine and libhwui.

Bug: 200309590
Ignore-AOSP-First: Introduces internal-only library
Test: builds
Change-Id: I611b79eb3addd15528f0cdb70e9f2f3d62473ec1