Provide credentials via Autofill when available

[sashei]

Acceptance Criteria

Lockbox-stored credentials are displayed on autofill-enabled pages when available. Selecting credentials fills the appropriate fields.

Note: This story does not deal with lock state. Provide credentials regardless of whether the app is locked or not.

Engineering notes

Handle incoming FillRequests and parse the FillContext to provide a FillCallback result of either all matching results or null if there are none.

Tasks

• Parse FillRequest for username and password fields
• Parse domain out of requesting app package name
• Search credentials for entries matching domain
• Display both username and domain for entry in autofill UI

Visual Design

InVision
Initial (1 entry): https://mozilla.invisionapp.com/share/3SN35UHWJDX#/317564412_Autofill_-_App_-_Initial
Initial (multiple entries): https://mozilla.invisionapp.com/share/3SN35UHWJDX#/317564409_Autofill_-_App_-_Initial_-multiple_Entries-
Filled: https://mozilla.invisionapp.com/share/3SN35UHWJDX#/317564408_Autofill_-_App_-_Filled

Zeplin
Initial (1 entry): https://zpl.io/2pBe6lE
Initial (multiple entries): https://zpl.io/aNo8g9v

Depends on #214