Make AM#isEnabled() true if any direct a11y connection exists.

Includes @TestApi AM#hasAnyDirectConnection() to improve test
reliability, because directly testing AM#isEnabled() is muddied
by UiAutomation used in the test itself.

Bug: 249164232
Test: atest AccessibilityEndToEndTest
Test: check node tree of a WebView when no real a11y service
      is enabled.
Change-Id: I4931ce316e9f5a956d266b7fd99f0faf00b8cd0d