refactor(appservice)!: Make USER_MEMBER const private"

[johannescpk]

No need to have it public anymore since the tests are now inside lib.rs.

Part of #228

[codecov]

Codecov Report

Merging #872 (1b426c1) into main (4b6a28d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #872   +/-   ##
=======================================
  Coverage   78.38%   78.38%           
=======================================
  Files         102      102           
  Lines       14223    14223           
=======================================
  Hits        11149    11149           
  Misses       3074     3074           

Impacted Files	Coverage Δ
crates/matrix-sdk-appservice/src/lib.rs	93.33% <ø> (ø)

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