Autofill compatibility mode.

Autofill helps users fill credentials, addresses, payment methods,
emails, etc without manually typing. When focus lands on a fillable
element the platform captures a snapshot of the screen content and
sends it to an autofill service for analysis and suggestions. The
screen snapshot is a structured representation of the screen content.
If this content is composed of standard widgets, autofill works
out-of-the-box. However, some apps do their own rendering and
the content in this case looks like a single view to the platform
while it may have semantic structure. For example, a view may render
a login page with two input test fields.

The platform exposes APIs for apps to report virtual view structure
allowing autofill services to handle apps that have virtual content.
As opposed to apps using standard widgets, this case requires the app
developer to implement the new APIs which may require a fair amount
of code and could be seen as a processes that could take some time.
The most prominent typs of apps that fall into this category are
browsers.

Until most apps rendering virtual content and specifically browsers
don't implement the virutal APIs, autofill providers need to fall-
back to using the accessibliity APIs to provide autofill support
for these apps. This requires developers to work against two sets
of APIs - autofill and accessibility - which is incovenient and error
prone. Also, users need to enable two plugins - autofill and
accessibility which is confusing. Additionally, the privacy and
perfomance impact of using the accessibility APIs cannot be addressed
while autofill providers need to use thes APis.

This change adds an autofill compatibility mode that would allow
autofill services to work with apps that don't implement the
virtual structure autofill APIs. The key idea is to locally enable
accessibility for the target package and remap accessibility to
autofill APIs and vise versa. This way an autofill provider codes
against a single set of APIs, the users enable a single plugin,
the privacy/performance implications of using the accessibility
APIs are addressed, the target app only takes a performance hit
since accessibility is enabled locally which is still more efficient
compared to the performance hit it would incur if accessibility is
enabled globally.

To enable compatibility mode an autofill service declares in its
metadata which packages it is interested in and also what is
the max version code of the package for which to enable compat
mode. Targeted versioning allows targeting only older versions of
the package that are known to not support autofill while newer
versions that are known to support autofill would work in normal
mode.

Since compatibility mode should be used only as a fallback we
have a white list setting with the packages for which this mode
can be requested. This allows applying policy to target only
apps that are known to not support autofill.

Test:
     cts-tradefed run cts-dev -m CtsAutoFillServiceTestCases
     cts-tradefed run cts-dev -m CtsAccessibilityServiceTestCases

bug:72811034

Change-Id: I11f1580ced0f8b4300a10b3a5174a1758a5702a0