Fixed ContentCaptureManager.isContentCaptureEnabled() when main session is disabled.

Each ContentCaptureManager has a MainContentCaptureSession associated with, and the main session
has a mDisabled state that's set to true when it failed to start (for example, because there's no
service associated with the user). Both objects used to share a common AtomicBoolean for the
disabled state, but a recent refactoring split then in a way that the manager's mDisabled was never
updated.

Test: atest ChildlessActivityTest#testGetContentCapture_enabledWhenNoService
Test: atest CtsContentCaptureServiceTestCases # sanityCheck

Bug: 123579223
Bug: 123307965
Bug: 123658889

Change-Id: Ib1f08f23721f208b28d0f339f39b21262b55e30d