docs: Adding N preview docs on Direct Boot feature
docs: Adding N preview docs on Direct Boot feature

Adding API overview summary and new topic on Direct Boot.
Direct Boot was formerly known as File-Based Encryption.
Adding changes based on review suggestions.

Bug: 26570782
Change-Id: Ibe253101c69e3c73122477c3af116e33fb8591a7

xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base