Reorganize public headers

[paulb777]

[READ] Step 1: Are you in the right place?

yes

[REQUIRED] Step 2: Describe your environment

• Xcode version: 8.3
• Firebase SDK version: 4.0
• Library version: 4.0
• Firebase Product: all (auth, database, storage, core, messaging, etc)

[REQUIRED] Step 3: Describe the problem

Consolidate public headers for each component into a single Public folder to make source management and podspec maintenance more robust.