Refactor register access from StackVisitor

Moves register access checking up to StackVisitor by adding methods
IsAccessibleGPR and IsAccessibleFPR in Context class. It allows to
simplify GetGPR/FPR and SetGPR/FPR methods in the Context class (and
its subclasses).

Also simplifies code in StackVisitor by adding IsAccessibleRegister,
GetRegister and SetRegister methods which then call either GPR or FPR
specific methods in Context depending on the nature of the accessed
register.

Bug: 18547544
Bug: 19106446
Change-Id: I6e707608d935a71571d0e975a6e766053de3763a