Fix GRPCCall refcounting issue

[muxi]

• When a call is to be finished, no further access to the same GRPCCall instance should be allowed since the self retain might be already gone and the object can be dealloc'ed.
• NSNotificationCenter is holding a weak reference to GRPCCall object. Make a strong reference in notification callback to make behaviors more predictable.

Fix #16045

[grpc-testing]

Objective-C binary sizes
*****************STATIC******************
  New size                      Old size
 1,980,459      Total (<)      1,980,471

 No significant differences in binary sizes

***************FRAMEWORKS****************
  New size                      Old size
10,810,803      Total (>)     10,810,797

 No significant differences in binary sizes

[grpc-testing]

Objective-C binary sizes
*****************STATIC******************
  New size                      Old size
 1,980,507      Total (>)      1,980,479

 No significant differences in binary sizes

***************FRAMEWORKS****************
  New size                      Old size
10,810,796      Total (>)     10,810,794

 No significant differences in binary sizes

[grpc-testing]

Objective-C binary sizes
*****************STATIC******************
  New size                      Old size
 1,981,758      Total (>)      1,981,730

 No significant differences in binary sizes

***************FRAMEWORKS****************
  New size                      Old size
10,814,016      Total (>)     10,814,015

 No significant differences in binary sizes

[muxi]

Interop ALTS Tests - #16203
InteropCloud-to-Cloud Tests - #13379