In the Google Drive app, within a Google document, there are two indentation keys in the top status bar. You'll need to scroll right to get to them. The right one works just like Tab, and the other is for Shift+Tab.
1. Put your cursor before the space you wish to indent
2. Hit enter to create a blank line of text between your first line and second line.
3. Click the "A" with the three horizontal lines next to it
4. Go to paragraph section
5. Click the button that has an arrow in the middle of the text pointing right (it usually will indent the whole portion/paragraph)
6. After doing this remove the space between the two lines that you made in step 2.
This works because it recognizes the first and second line as separate when you format it.