Fix the context leak in the ContentCapture

Activity cannot be released because the context be holded by multiple
objects.
* Activity holds ContentCaptureManager.
* ContentCaptureManager holds the context and MainContextCaptureSession.
* MainContextCaptureSession holds the context and ContentCaptureManager.
* The system server holds some binder references to MainContentCaptureSession.
If the system service never released the binder references, then the
activity is also never GC'd.
To avoid the issue,
1. Make the session state receiver of MainContextCaptureSession to be
   static and uses weak reference to MainContextCaptureSession.
2. The direct service vulture may miss to do unlinkToDeath(), do a checking
   while the session destory.

Bug: 143210612
Test: manual check Objects on the meminfo
Test: Activities should be released after a period of time
Test: adb shell dumpsys meminfo com.google.android.dialer
Change-Id: I12037483addb1efe444c74fa189ef6afd15821dd