Update ShaderCache to support shared locking

Currently, the shadercache can lock for a long time when writing the
cache to disk while vulkan is trying to access it, causing frame drops.
However, neither operation actually requires write access. By using
shared_locks, both operations can happen concurrently eliminating this
contention and saving the frame(s).

Test: atest hwui_unit_tests:ShaderCacheTest
Bug: 288252750

Change-Id: I26c5bee001d44cdd75766e9cb5974bd9337819ab