Make TArray safer to use and update method name for consistency.

[edgchen1]

Description
Update TArray:

• make size_ and data_ data members private
• rename GetCapacity() to Capacity() to be consistent (e.g., with Size())
• add static_assert for trivially copyable T because it is copied with memcpy

Motivation and Context
The original potential issue was that size_ could be set to any value, including beyond the capacity of the TArray. This could result in out of bounds memory access. Now the invariant that size_ <= capacity is enforced.