docs: updates to Buildilng Your First App doc

This commit includes the following updates:
- replaces Basic activity with Empty activity
- uses default values wherever possible
- removes instructions for floating button
- removes SDK instructions. Users must use Android Studio
- updates the SDK version to API 15
- combines some instructions that were too text heavy

Bug: 29766869
Bug: 29768686
Bug: 19154557

Change-Id: Id819cb2dde74b68b5145dfefe7a8502892bb377a