docs: Fixing various minor issues

Fixes to minor issues (typos and other things that don't require
editorial review) in Android docs.

Staged page URLs provided in CL comments.

Bug: 28172346
Bug: 22411462
Bug: 26422744
Bug: 26776224
Change-Id: Ic3aa8495a1d76a5ce3bccb5037b4172443664d0b