Swaps isBase64 with internal implementation for Android compatibility

[dotasek]

Fixes #1046 by duplicating a portion of the newer Apache codec api.

Differs from the similar PR #1051 in that it doesn't use a deprecated method.

[codecov]

Codecov Report

Base: 9.93% // Head: 9.93% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (32857c5) compared to base (a24add4).
Patch coverage: 91.66% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1079   +/-   ##
=========================================
  Coverage      9.93%    9.93%           
  Complexity    13200    13200           
=========================================
  Files          2483     2484    +1     
  Lines        976377   976387   +10     
  Branches     290293   290294    +1     
=========================================
+ Hits          96988    97010   +22     
+ Misses       842233   842223   -10     
+ Partials      37156    37154    -2     

Impacted Files	Coverage Δ
...s/src/main/java/org/hl7/fhir/utilities/Base64.java	90.00% <90.00%> (ø)
.../java/org/hl7/fhir/r4b/model/Base64BinaryType.java	80.00% <100.00%> (ø)
...n/java/org/hl7/fhir/r5/model/Base64BinaryType.java	82.85% <100.00%> (ø)
...java/org/hl7/fhir/r5/context/TerminologyCache.java	69.09% <0.00%> (+0.24%)	⬆️
...a/org/hl7/fhir/r5/test/utils/CompareUtilities.java	59.15% <0.00%> (+1.87%)	⬆️
.../fhir/utilities/tests/CacheVerificationLogger.java	20.83% <0.00%> (+8.33%)	⬆️
...c/main/java/org/hl7/fhir/utilities/FileFormat.java	77.77% <0.00%> (+66.66%)	⬆️

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