docs: Fixing malformed @links

Several @link javadoc directives were malformed, resulting in the
Javadoc code being output to the HTML.

Staged to:

http://go/dac-stage/reference/android/app/Activity
http://go/dac-stage/reference/android/app/RemoteInput
http://go/dac-stage/android/os/UserManager

Bug: 120743732
Test: make ds-docs
Change-Id: I2c00c7e1e3adcff696e6d000206aeeaea61e7ecd
Exempt-From-Owner-Approval: Doc-only change