Fix HWUI ShaderCache mIDHash eviction race

This patch fixes the ShaderCache eviction race by ensuring the ID
remains in the cache outside of the background thread, during set().

This patch also makes set() a class method, and removes
getBlobCacheLocked() for simplicity as it no longer serves to
"ensure the blob cache is initialized" as initShaderDiskCache() handles
that role now.

Test: atest hwui_unit_tests:ShaderCacheTest
Bug: 290231463
Change-Id: Id69aff95a0cf3bed6b3b7dfc2bd61b2a8a2b555a