Removed single use of session holder and decouple drop in implementation

[github-actions[bot]]

LCOV of commit 2200899 during Check PR #549

Summary coverage rate:
  lines......: 1.8% (38 of 2153 lines)
  functions..: no data found
  branches...: no data found

Files changed coverage rate:
                                                                                                |Lines       |Functions  |Branches    
  Filename                                                                                      |Rate     Num|Rate    Num|Rate     Num
  ====================================================================================================================================
  lib/src/drop_in/drop_in.dart                                                                  | 0.0%    131|    -     0|    -      0
  lib/src/drop_in/drop_in_platform_api.dart                                                     | 0.0%     14|    -     0|    -      0
  lib/src/generated/platform_api.g.dart                                                         | 0.8%   1023|    -     0|    -      0

[github-actions[bot]]

LCOV of commit 82775ad during Check PR #550

Summary coverage rate:
  lines......: 1.8% (38 of 2153 lines)
  functions..: no data found
  branches...: no data found

Files changed coverage rate:
                                                                                                |Lines       |Functions  |Branches    
  Filename                                                                                      |Rate     Num|Rate    Num|Rate     Num
  ====================================================================================================================================
  lib/src/drop_in/drop_in.dart                                                                  | 0.0%    131|    -     0|    -      0
  lib/src/drop_in/drop_in_platform_api.dart                                                     | 0.0%     14|    -     0|    -      0
  lib/src/generated/platform_api.g.dart                                                         | 0.8%   1023|    -     0|    -      0

[sonarcloud[bot]]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarCloud